Improving Email Deliverability: The Essential Guide to List Management

You are paying Klaviyo to email 60,000 people who do not remember your brand. That is not a deliverability problem. That is a list management problem wearing a deliverability problem's coat. Every week, a founder or marketing director slides into my DMs with a variation of the same message: "Our emails are going to spam. Open rates are collapsing. We've tried everything — new sending domain, new templates, new subject lines. Nothing's working." They have tried everything except the one thing that actually fixes it. The answer is not a new domain. The answer is not a better subject line. The answer is that you have been emailing 80,000 people, 30,000 of whom have not opened a single email in 18 months, and mailbox providers — Gmail, Outlook, Yahoo, Apple Mail — have noticed. They have been noticing for months. And they have been quietly routing every email you send a little further from the inbox and a little closer to the spam folder, silently, without telling you, while your Klaviyo dashboard shows you an "open rate" calculated against a denominator that includes thousands of contacts who will never open anything you send again.

The unsexy truth about email deliverability is that it is almost never a technical problem in the way most brands assume. SPF, DKIM, and DMARC matter — they are the foundation, and if they are not configured correctly, nothing else you do will help. But in 99% of the deliverability cases I audit, the DNS records are fine. The issue is behavioural. Mailbox providers do not punish you for sending too frequently. They punish you for sending to people who do not want it. And the clearest signal that someone does not want your email is that they have not opened one in six months. When 30–40% of your list consistently ignores you, the algorithms read that silence as a spam signal — and they apply that judgment to your entire sending reputation, including the emails going to the 20,000 subscribers who do want to hear from you and would buy from you if only your email reached their inbox.

This post is the full technical breakdown of the list hygiene and sunset flow architecture we run on every new client in week one. Not a summary. Not a framework overview. The actual configuration — the suppression thresholds, the sunset email sequence, the re-engagement logic, the Klaviyo segment architecture, the DNS verification checklist, and the post-cleanup monitoring protocol. The list you should care about is not the one you have. It is the one that opens.

How Mailbox Provider Reputation Scoring Actually Works

Most brand-side marketers conceptualise deliverability as a binary: either your email lands in the inbox or it goes to spam. The actual model is more nuanced and more punishing. Mailbox providers — primarily Google, Microsoft, and Apple, who collectively control the inbox for roughly 85% of email addresses — maintain a reputation score for every sending domain and every sending IP address. This score is not static. It is a rolling average of engagement signals, recalculated continuously as new sends go out and new engagement data comes in.

The signals that increase your reputation score are the ones you would expect: opens, clicks, replies, forwards, and saves. The signals that damage it are subtler: not-opens (a subscriber who receives your email and does not open it), spam complaints (the "report spam" button), and unsubscribes (which are better than spam complaints but still negative signals at high volume). The signal that most brands fail to account for is the sustained pattern of not-opens from a large segment of their list. Gmail's spam filter does not fire after one ignored email. It fires when the pattern is persistent — when a contact receives 20 emails over 6 months and opens zero of them. At that point, Gmail has high statistical confidence that this contact does not want your email, and it routes accordingly.

The mechanism that makes this catastrophic at scale is domain-level reputation aggregation. Google does not evaluate each email individually against each recipient's engagement history in isolation. It aggregates the engagement pattern from your entire sending domain and applies a reputation score that affects inbox placement for all sends from that domain, including your sends to highly engaged subscribers. When 35% of your list is chronically disengaged, that 35% is actively dragging down the inbox placement of the 65% who do want to hear from you. You are paying for the engaged subscribers to be penalised by the presence of the disengaged ones.

"You are not fighting Gmail. You are fighting your own list. Every unengaged subscriber you keep is a vote against yourself in the algorithm that decides whether your best customers see your email." Arsalan Faysal — Revenue Systems Architect

MAILBOX PROVIDER REPUTATION MODEL — HOW GMAIL SCORES YOUR DOMAIN
──────────────────────────────────────────────────────────────────

POSITIVE SIGNALS (increase domain reputation score):
  + Open within 2 hours of send        (strong positive signal)
  + Click on a link                     (strong positive signal)
  + Reply to the email                  (very strong positive — rare but weighted heavily)
  + Forward to another address          (strong positive)
  + Move from Promotions to Primary     (very strong — manual user action)
  + Add sender to contacts              (very strong)
  + Mark as "not spam" after filter     (recovery signal)

NEGATIVE SIGNALS (decrease domain reputation score):
  − Email received, never opened        (mild but compounds severely at scale)
  − Unsubscribe                         (moderate negative)
  − Spam complaint via "Report Spam"    (severe — most damaging single signal)
  − Hard bounce (invalid email)         (severe — indicates poor list hygiene)
  − Soft bounce pattern (repeated)      (moderate — signals list decay)
  − Move from inbox to spam manually    (severe)

REPUTATION AGGREGATION MECHANICS:
  → Gmail evaluates domain reputation at the sending IP + domain level
  → Rolling 30-day and 90-day engagement windows weighted differently
  → Not-open rate from disengaged segment pulls down ENTIRE domain score
  → Spam complaint rate above 0.10% triggers active filtering throttling
  → Spam complaint rate above 0.30% triggers near-universal spam routing
  → Recovery from damaged reputation: minimum 90 days of clean sending

Week-One Diagnostic: Reading Your Klaviyo Data Like a Reputation Audit

Before touching a single suppression or building a single sunset flow, you need an accurate picture of your list's health across four dimensions. Most brands look at their overall open rate and conclude that deliverability is or is not a problem. The overall open rate is the least useful number in your Klaviyo account for diagnosing deliverability. Here is the diagnostic framework we run before any remediation work begins.

The Four-Segment Engagement Breakdown

Pull four segments in Klaviyo — no filters, no exclusions, just raw counts — and record the numbers. These four segments tell you the actual state of your list with enough precision to quantify the severity of the problem and the expected impact of suppression.

Spam Complaint Rate — The Number That Overrides Everything

Pull your spam complaint rate from Google Postmaster Tools — not from Klaviyo. Klaviyo's spam complaint data is incomplete because most email clients do not route spam complaint feedback back to ESPs. Google Postmaster Tools gives you the direct signal from Gmail's infrastructure. Register your sending domain at postmaster.google.com and monitor the Domain Reputation and Spam Rate dashboards. These are not estimates — they are the actual numbers Gmail uses when deciding where to place your emails.

The Sunset Flow Architecture: Three Emails Before You Say Goodbye

The sunset flow is the mechanism that separates a professional list hygiene operation from an aggressive mass-suppression that discards potentially recoverable subscribers. The logic is simple: before suppressing any contact as disengaged, give them one final, explicit, honest opportunity to re-engage. Three emails. Spaced over 10 days. Designed with maximum simplicity and minimum friction. Anyone who clicks anything in those three emails gets moved back to the active list. Everyone who does not gets suppressed. The suppression is permanent unless the contact re-subscribes through an active consent point.

Sunset Flow Entry Trigger and Enrollment Logic

SUNSET FLOW — KLAVIYO CONFIGURATION
──────────────────────────────────────────────────────────────────

ENROLLMENT TRIGGER:
  Person is in segment: "At Risk — No Open in 91–180 Days"
  AND Person is NOT already in a sunset flow sequence
  AND Person is NOT suppressed

FLOW FILTER (checked at each step):
  IF person opens OR clicks any email in the sequence:
    → EXIT flow immediately
    → Add to segment: "Re-Engaged via Sunset Flow"
    → Remove from "At Risk" segment
    → Add to "Engaged" segment (last active = today)
    → Fire internal notification: "Re-engagement win: [email]"

IF person completes all 3 emails with zero engagement:
  → Add to Klaviyo suppression list (Marketing Suppressed)
  → Tag contact: suppressed_sunset_[YYYY-MM]
  → Remove from all active campaign lists
  → Log suppression date as custom property: sunset_suppressed_date

TIMING:
  Email 1 → Send immediately on enrollment
  Email 2 → Wait 4 days after Email 1
  Email 3 → Wait 6 days after Email 2
  Suppression → 48 hours after Email 3 (if no engagement)

Sunset Email 1 — The Simple Check-In

The first email in the sunset flow is not a sale, not a promotion, and not a "we miss you" attempt dressed in guilt-trip language. It is a direct, honest acknowledgement that you have noticed they have not been engaging, and a simple, frictionless offer to either re-engage or officially opt out. The subject line should be the simplest it can be. The body copy should be the shortest it can be. Every design element that is not a click target is noise.

Sunset Email 2 — The Stakes Email

Four days after Email 1, if there has been zero engagement, Email 2 goes out. The framing shifts slightly: you are now telling the contact what is about to happen, not asking what they want. The purpose is to trigger the loss-aversion response — people are more motivated by the prospect of losing access to something than by the prospect of gaining it. If your brand has delivered any real value to these subscribers in the past, this email gives them a concrete reason to act before the window closes.

Sunset Email 3 — The Final Notice

Email 3 is the last email this contact will ever receive from you unless they actively click to stay. Send it 6 days after Email 2. The subject line is the most direct it has been in the entire sequence. The body is shorter than Email 1. There is one call to action — "Don't remove me" — and nothing else. No unsubscribe link is needed at this point because the suppression happens automatically if they do nothing. Some legal frameworks require an unsubscribe link on all commercial emails regardless of context — confirm your jurisdictional requirements and add a plain-text unsubscribe at the bottom if required.

"The brands 'fighting deliverability' are the ones who treat every unsubscribe as a loss. An unsubscribe is free list hygiene. A spam complaint is expensive. A chronically unengaged subscriber who never unsubscribes is the most expensive thing on your list — they cost you your inbox placement with everyone else." Arsalan Faysal — Revenue Systems Architect

Klaviyo Technical Configuration: Segments, Flow Logic, and Suppression Architecture

The sunset flow concept is straightforward. The Klaviyo implementation requires precision. The segment definitions, flow filter logic, and suppression actions must be configured in a specific way to prevent the most common failure modes: contacts who re-engage being suppressed anyway, contacts being enrolled in the sunset flow multiple times, and suppressed contacts being re-added to active lists by future imports or integration syncs.

The Six Klaviyo Segments You Need to Build

KLAVIYO SEGMENT ARCHITECTURE — LIST HEALTH MANAGEMENT SYSTEM
──────────────────────────────────────────────────────────────────

SEGMENT 1: "Active — Highly Engaged"
  Filter: Someone who has opened email in last 30 days
  AND (opened email OR clicked email in last 30 days)
  Update frequency: Real-time (Klaviyo default)
  Use: Core sending list. All campaigns should include this segment.

SEGMENT 2: "Active — Engaged"
  Filter: Someone who has opened email in last 31–90 days
  AND has NOT opened email in last 30 days
  Update frequency: Real-time
  Use: Send campaigns but monitor open rate. 
       If open rate drops below 15%, enroll in re-engagement nurture.

SEGMENT 3: "At Risk — Sunset Candidate"
  Filter: Someone who has received at least 3 emails in last 90 days
  AND has NOT opened email in last 90 days
  AND has NOT opened email in last 180 days (catches the full window)
  AND is NOT suppressed
  AND does NOT have property: sunset_suppressed_date (has value)
  Update frequency: Real-time
  Use: Sunset flow enrollment trigger. 
       DO NOT include in any regular campaigns while in this segment.

SEGMENT 4: "Re-Engaged via Sunset Flow"
  Filter: Someone who has property: sunset_re-engaged = true
  Update frequency: Real-time
  Use: Track recovery rate from sunset flow. 
       These contacts should receive a welcome-back email 
       before being merged back into Active — Engaged segment.

SEGMENT 5: "Suppressed — Sunset"
  Filter: Someone who has property: sunset_suppressed_date (has value)
  Update frequency: Real-time
  Use: Audit and reporting only. Do NOT send campaigns. 
       Do NOT import into any active lists from here.

SEGMENT 6: "Never Opened — New Subscriber"
  Filter: Someone who has subscribed in last 30 days
  AND has NOT opened any email ever
  Update frequency: Real-time
  Use: Monitor new subscriber activation rate. 
       If someone subscribes and opens nothing in 30 days, 
       they need a different welcome sequence — not suppression yet.

Flow Filter Logic — Preventing Double-Enrollment and Suppression of Re-Engagers

The most critical technical detail in the sunset flow configuration is the flow filter that prevents contacts from being suppressed after they have re-engaged. Klaviyo's flow logic evaluates filters at each step by default — but only if you configure them correctly. Without an explicit real-time check at the suppression step, a contact who opens Email 2 but whose re-engagement event has not yet been processed by Klaviyo's segmentation engine could be suppressed by the day-12 automation action.

The DNS Foundation: SPF, DKIM, and DMARC — Non-Negotiable Before Any List Work

The list hygiene work above assumes your technical sending infrastructure is correctly configured. If your SPF, DKIM, and DMARC records are missing or incorrectly set up, mailbox providers cannot verify that your emails are legitimately sent from your domain — and no amount of list hygiene will fix a fundamental authentication failure. Verify these records before running any sunset flow or suppression campaign. If they are misconfigured and you suppress your entire disengaged list, your deliverability will improve marginally — but the underlying authentication failure will cap your inbox placement regardless of how clean your list is.

The Revenue Math: Why a Smaller List Makes You More Money

The CFO objection is always the same: "We're paying for 80,000 contacts and you want to suppress 40,000 of them? We're cutting our list in half — how does that make business sense?" The answer is in the unit economics, and once you run the numbers it is not a difficult argument to make.

LIST HYGIENE REVENUE IMPACT MODEL
──────────────────────────────────────────────────────────────────

BEFORE SUPPRESSION: 80,000 active contacts

  Total sends per campaign:           80,000
  Open rate (current):                18%
  Opens per campaign:                 14,400
  Click rate (% of opens):            2.5%
  Clicks per campaign:                360
  Conversion rate (% of clicks):      3.5%
  Purchases per campaign:             12.6
  Average order value:                $120
  Revenue per campaign:               $1,512
  Revenue per email sent:             $0.019

AFTER SUPPRESSION: 44,000 active contacts (45% suppressed)

  Total sends per campaign:           44,000
  Open rate (post-suppression):       36%
  Opens per campaign:                 15,840    ← MORE opens than before
  Click rate (% of opens):            3.2%      ← Higher: engaged audience
  Clicks per campaign:                507
  Conversion rate (% of clicks):      4.1%      ← Higher: intent-matched audience
  Purchases per campaign:             20.8
  Average order value:                $120
  Revenue per campaign:               $2,496
  Revenue per email sent:             $0.057    ← 3x revenue per email sent

NET IMPACT:
  List size reduction:                -45%
  Klaviyo cost reduction:             -45% (contacts-based pricing)
  Revenue per campaign increase:      +65%
  Revenue per email sent increase:    +200%
  Spam complaint rate change:         -60 to -80% (disengaged contacts drive complaints)
  Domain reputation trajectory:       Recovery begins within 30 days of suppression

The Klaviyo cost reduction alone often covers the engagement of a specialist to run the cleanup. Klaviyo's pricing is contact-based — suppressing 36,000 contacts from an 80,000-contact list moves a typical brand from the $700/month plan to the $400/month plan. Over 12 months, that is $3,600 saved on software that was being used to damage your sender reputation. The revenue increase from improved inbox placement compounds that number further: more opens from the same engaged audience, higher click rates because engaged subscribers have self-selected for relevance, and higher conversion rates because you are hitting people who have a demonstrated purchase history with your brand.

"The list you should care about is not the one you have. It is the one that opens. A 44,000-contact list that reaches the inbox is worth more than an 80,000-contact list that hits the Promotions tab — and it costs you less to send to." Arsalan Faysal — Revenue Systems Architect

Post-Suppression Monitoring Protocol: The 90-Day Recovery Tracking Stack

Suppression day is not the end of the work. It is the beginning of the measurement phase. Domain reputation recovery from a previously damaged state takes a minimum of 30 days of consistent clean sending, and the full recovery to stable inbox placement in Gmail Primary (for brands that were previously in Promotions) takes 60–90 days. During this window, the monitoring protocol is tighter than normal operating rhythm — you are watching for signals that confirm recovery is on track and catching any regression before it compounds.

Ongoing List Hygiene: The System That Prevents You From Being Here Again

The sunset flow and suppression campaign solve the acute problem. The ongoing hygiene system prevents the same problem from rebuilding itself over the next 12–18 months. Without an automated maintenance system, a brand that runs a clean suppression today will be back to the same disengagement ratio within 18 months — because new subscribers who never activate accumulate, because engaged subscribers drift into inactivity without a re-engagement trigger, and because no one on the marketing team is manually auditing the segment health week after week.

The maintenance system is three automated Klaviyo flows running in the background continuously: a New Subscriber Activation Flow that monitors whether new subscribers open anything in their first 30 days and triggers a different welcome sequence if they do not; a Rolling Engagement Monitoring Flow that checks all active subscribers at the 60-day no-open mark and enrolls them in a light re-engagement sequence before they hit the 90-day At Risk threshold; and the Sunset Flow itself, which now runs as a permanent background automation rather than a one-time campaign — automatically catching any subscriber who drifts to the 91-day no-open threshold and processing them through the three-email sequence before suppression.

Run these three flows permanently. Audit your four engagement segments monthly. Check Google Postmaster Tools weekly. And resist the pressure — from your CFO, from your growth team, from any agency that measures success by list size — to grow your subscriber count faster than you grow your engagement rate. A 20,000-contact list with a 40% open rate is a more valuable email channel than an 80,000-contact list with an 18% open rate. The math does not change because the number looks smaller. The revenue per send goes up. The Klaviyo bill goes down. And the inbox placement holds — because the algorithm sees what it needs to see: a sender whose subscribers actually want to hear from them.