Deliverability is the multiplier on every email you send. A well-built flow that lands in spam earns nothing, and a reputation problem usually shows up as a revenue problem first: flat opens, falling clicks, and flows that quietly underperform. This guide explains how mailbox providers judge your emails and gives a step-by-step plan to diagnose and repair sender reputation.
The short version: deliverability comes down to three things: you are who you say you are (authentication), people want your emails (engagement and consent), and you send in a pattern providers trust (volume, frequency, and list quality).
How mailbox providers decide where your email goes
Gmail, Yahoo, and Microsoft look at the same broad signals:
- Authentication: do SPF, DKIM, and DMARC pass and align with your domain?
- Engagement: do recipients click, reply, and read, or delete and ignore?
- Complaints: how often do recipients mark you as spam?
- List quality: are you hitting bounces, invalid addresses, or spam traps?
- Consistency: is your volume steady, or does it spike unpredictably?
AI features in Gmail and Apple Mail add another layer of relevance ranking on top of these signals. See AI and the Inbox.
The current sender requirements
Google and Yahoo introduced bulk sender requirements in 2024, and Microsoft followed in 2025. Third-party coverage describes all three as strictly enforced by 2026. Google defines bulk senders as those sending around 5,000 or more messages a day to Gmail addresses, but the practices apply at any volume.
| Requirement | What it means | What to do |
|---|---|---|
| Authentication | SPF and DKIM must pass; a DMARC record must be published | Set up and verify all three on your sending domain |
| One-click unsubscribe | Marketing email needs a header-based one-click option (RFC 8058) plus a visible link, honored promptly | Confirm your platform adds it; Klaviyo handles the header for marketing sends, but verify |
| Low spam complaint rate | Stay under 0.3%; aim for under 0.1% | Track in Google Postmaster Tools and your platform |
DMARC expectations are tightening. Some sources report that a policy of p=none may no longer satisfy requirements and that stricter policies (quarantine or reject) are likely. Check the current requirements from Google, Yahoo, and Microsoft directly before you set a policy, and move toward enforcement gradually so you do not block legitimate mail.
Signs you have a deliverability problem
- Open and click rates dropped without a change in content or segmentation
- Revenue from flows declined while list size grew
- Complaint rates or bounces are rising
- Gmail users see lower engagement than other providers
- Postmaster Tools shows low domain or IP reputation
- Customers tell you they never received an email
Open rate alone is a weak signal because Apple Mail and Gmail features inflate it. Look at clicks, placed orders, and provider-level trends.
The diagnosis checklist
| Check | Where to look | Healthy sign |
|---|---|---|
| Authentication | DNS records, your platform's sending domain settings | SPF, DKIM, and DMARC all pass and align |
| Spam complaints | Google Postmaster Tools, platform reports | Under 0.1% ideally, never near 0.3% |
| Bounce rate | Platform reports | Low and stable; hard bounces suppressed |
| Engagement by provider | Segment by Gmail, Yahoo, Outlook, Apple | No sharp drop at one provider |
| List acquisition quality | Sign-up source reports | No spikes from bots, giveaways, or low-intent sources |
| Sending patterns | Volume by day and segment | No sudden spikes to cold segments |
| Content | Spam testing, link and image checks | Clean links, balanced text and images |
Step 1: Fix the foundation
- Authenticate your domain. Set up SPF, DKIM, and DMARC. Use a dedicated sending subdomain if your platform supports it.
- Check alignment. The domain in your From address should align with your authenticated domain.
- Enable one-click unsubscribe, and keep a visible unsubscribe link in the body.
- Register with Google Postmaster Tools so you can see domain reputation and spam rate.
- Consider BIMI to show your logo, which supports recognition and trust.
Step 2: Clean your list
A large list full of disengaged addresses drags down reputation.
- Suppress hard bounces and invalid addresses
- Remove or quarantine bot sign-ups and obviously fake emails
- Define unengaged by clicks and purchases over a window that fits your sales cycle
- Run a re-engagement series, then suppress those who do not respond
- Add bot protection to sign-up forms, and consider double opt-in if bot sign-ups are a problem
Step 3: Send to people who want it
Engagement is the strongest long-term signal.
- Send to engaged segments first, then widen carefully
- Reduce frequency for unengaged subscribers
- Make promotions relevant by segment instead of blasting the full list
- Prioritize flows, since behavior-triggered messages tend to be more relevant
- Make unsubscribe easy, because a friction-free exit prevents spam complaints
Step 4: Warm up and ramp carefully
When you change domains, add a new sending identity, or restart after a long gap, ramp up volume gradually.
| Phase | Audience | Goal |
|---|---|---|
| Start | Most engaged subscribers (recent clickers and buyers) | Build positive signals |
| Build | Engaged subscribers from the last 60 to 90 days | Expand while watching complaints and bounces |
| Expand | Broader active list | Return to normal volume |
| Maintain | Full program with sunset policy | Hold steady |
Watch complaints and bounces at every step, and slow down if either rises.
Step 5: Repair a damaged reputation
If you are already in trouble:
- Pause non-essential promotions to unengaged segments right away
- Send only to your most engaged subscribers for two to four weeks
- Keep flows running for engaged recipients, since they send high-relevance messages
- Fix the root cause. Common causes are a bad acquisition source, a sudden volume spike, a list import, or too-frequent sending
- Monitor weekly in Postmaster Tools and your platform, and widen the audience only when metrics improve
- Keep records of changes and results so you can see what worked
Recovery takes time. Expect weeks, not days, and avoid quick fixes that promise instant results.
Deliverability and peak season
Volume spikes around BFCM are a common trigger for reputation problems. Start warming engaged segments early, and ramp gradually rather than jumping to peak volume. See BFCM 2026 Retention Planning Guide.
Platform-specific notes for Klaviyo
- Use a dedicated sending domain and confirm authentication in your account settings
- Use engagement-based segments for campaigns, and avoid sending promotional campaigns to your whole list every time
- Review Smart Sending and frequency settings so you are not over-messaging
- Check the sunset flow and make sure it suppresses unengaged profiles
- Monitor deliverability metrics in your account and external tools
For migrations, deliverability planning is critical. See Klaviyo Migration Checklist.
Common deliverability mistakes
- Importing old lists without permission or recent engagement
- Sending to the full list for every campaign
- Ignoring complaint rates until there is a problem
- Buying or renting email lists
- Changing domains or platforms right before a peak season
- Hiding the unsubscribe link
Deliverability checklist
- ☐ SPF, DKIM, and DMARC pass and align
- ☐ One-click unsubscribe works, with a visible link
- ☐ Google Postmaster Tools connected
- ☐ Complaint rate tracked and kept under 0.1%
- ☐ Hard bounces suppressed automatically
- ☐ Sunset flow live
- ☐ Engagement-based segmentation used for campaigns
- ☐ Warm-up plan in place for any change in domain or volume
- ☐ Monthly deliverability review scheduled
FAQ
Why are my emails going to spam?
Common reasons are failing authentication, high complaint rates, low engagement, poor list quality, or sudden volume spikes.
What spam complaint rate is acceptable?
Google guidance referenced by industry sources says to stay under 0.3%, with under 0.1% as the target.
Do small senders need to follow bulk sender rules?
The formal thresholds apply at around 5,000 messages a day, but the practices help at any size.
How long does it take to fix sender reputation?
Typically weeks, depending on how severe the problem is and how quickly you change sending patterns.
Can an agency fix deliverability?
An agency can diagnose causes, build a recovery plan, and manage the ramp, but the underlying list and sending habits have to change.
How ATTN helps
ATTN diagnoses and repairs deliverability for ecommerce brands on Klaviyo, from authentication and list hygiene to warm-up and engagement-based sending. Talk to our retention team about a deliverability audit.