If deliverability is dropping now, treat it as a reputation problem first and a list-size problem second. The major mailbox providers increasingly care about recipient engagement, spam complaints, authentication, and sending consistency.
1. Tighten list hygiene
- Stop mailing unengaged contacts. Create an engagement segment—e.g. people who opened/clicked recently—and temporarily suppress older inactive subscribers. Google explicitly recommends considering removal of people who don't open or read your messages.
- Never buy, rent, or append addresses. Mail only to people who explicitly opted in.
- Use confirmed/double opt-in where practical, particularly for high-risk acquisition sources.
- Suppress hard bounces immediately and suppress repeated soft bounces rather than continually retrying.
- Remove role accounts, disposable addresses, spam traps, and obviously invalid addresses using validation before they enter your active marketing pool.
- Don't treat "hasn't opened in 90 days" as an automatic universal rule. Instead, test a re-engagement sequence and then suppress people who remain inactive.
- Make unsubscribing extremely easy. Fighting an unsubscribe by continuing to mail someone is usually worse for reputation than losing that subscriber.
2. Get authentication exactly right
For a serious sending program, aim for:
SPF + DKIM + DMARC, with DMARC alignment between your visible From: domain and the authenticated domain.
Google currently requires SPF or DKIM for all senders, and SPF, DKIM and DMARC for bulk senders. Bulk promotional mail also needs RFC 8058 one-click unsubscribe.
Also check:
- DKIM is passing consistently.
- Your SPF record includes every legitimate sending service and isn't unnecessarily fragmented.
- DMARC is receiving reports and you're investigating failures.
- Your sending IP has valid forward/reverse DNS.
- TLS is being used.
- Your ESP isn't inadvertently rewriting authentication or alignment.
If you're currently at p=none, don't jump blindly to quarantine/reject; first use DMARC reporting to identify legitimate senders and fix alignment.
3. Protect the domain's reputation
Think of reputation as domain + IP + traffic type + recipient behavior rather than one single score.
- Keep transactional and marketing mail separated where possible—different streams/subdomains/IP pools can prevent promotional problems from contaminating critical mail.
- Keep the same type of traffic associated with consistent
From: identities.
- Avoid sudden volume spikes. Google recommends consistent sending rates and gradually increasing volume, particularly when scaling.
- If reputation has deteriorated, don't simply switch to a new domain/IP and continue the same behavior. That tends to recreate the problem and can look suspicious.
- Avoid campaigns that generate a sudden burst of complaints from a poorly engaged segment.
4. Watch the metrics that actually matter
I'd put these on a daily/weekly dashboard:
| Metric | What I'd watch |
|---|
| Spam complaints | Primary danger signal |
| Hard bounce rate | Data/acquisition quality |
| Inbox placement | Actual deliverability, not merely "delivered" |
| Domain reputation | Especially Gmail |
| IP reputation | Important for dedicated/shared infrastructure |
| Engagement by mailbox | Gmail, Microsoft, Yahoo, etc. separately |
| Unsubscribe rate | Content/list-fit signal |
| Deferrals / SMTP errors | Early warning of provider throttling |
For Gmail specifically, Google says senders should keep the reported spam rate below 0.1% and avoid reaching 0.3% or higher.
Also use Google Postmaster Tools to inspect domain/IP reputation, authentication, spam rate, and delivery-related signals.
5. Implement one-click unsubscribe
For marketing/promotional mail, don't rely solely on a footer link. Gmail's bulk-sender requirements call for RFC 8058 List-Unsubscribe / one-click unsubscribe, alongside a visible unsubscribe mechanism. Google recommends honoring requests within 48 hours.
If you're already seeing a decline
I'd do this in order:
- Pause the least-engaged segment.
- Check Gmail/Postmaster and other mailbox-provider reputation signals.
- Examine complaint and bounce rates by campaign, acquisition source, domain, and sending stream.
- Audit SPF/DKIM/DMARC alignment.
- Verify one-click unsubscribe.
- Resume with your most engaged recipients first, at a stable volume.
- Gradually reintroduce less-engaged segments only if reputation recovers.
- Fix the acquisition/content problem causing complaints before trying to scale again.
One important current development: Gmail says it ramped up enforcement on non-compliant traffic starting in November 2025, including temporary and permanent rejection for failures to meet sender requirements.
If you tell me your monthly send volume, ESP (e.g. SendGrid/Klaviyo/Mailchimp/SES), approximate complaint/bounce rates, and whether the decline is mainly Gmail, Outlook, Yahoo, or everywhere, I can help pinpoint the likely failure mode.
