Email deliverability
Why mail lands in spam, and the handful of things that actually fix it.
On this page
The three things that matter most
1. Authentication. SPF, DKIM, and DMARC must all pass. Without them, Gmail and Outlook increasingly reject bulk mail outright rather than filing it in spam. See Verify your sending domain.
2. Engagement. Mailbox providers watch whether recipients open, reply, and don't delete unread. Sending to people who ignore you trains the filters against you.
3. Complaint rate. Spam complaints above roughly 0.3% cause real damage. Above 0.5% you will be throttled or blocked.
Everything else is a rounding error by comparison.
Warming up a new domain
A domain with no sending history that suddenly sends tens of thousands of emails looks exactly like a compromised domain. Ramp up:
| Week | Daily volume |
|---|---|
| 1 | 200 |
| 2 | 1,000 |
| 3 | 5,000 |
| 4 | 20,000 |
| 5+ | Full volume |
Send to your most engaged contacts first. Early positive engagement is what builds the reputation that carries the later volume.
Bounces
Hard bounces — the address does not exist. Remove immediately; repeatedly mailing dead addresses is a strong negative signal.
Soft bounces — mailbox full, server temporarily down. Retried automatically; consistent soft bouncing over weeks should be treated as hard.
Content that triggers filters
- ALL CAPS SUBJECT LINES
- Excessive exclamation marks
- "Free", "guaranteed", "act now" stacked together
- A single large image with almost no text
- Link shorteners — use your own domain
- Mismatched from-name and from-address
None of these matter individually as much as authentication and engagement, but together they push a borderline message over the line.
Diagnosing a problem
- Send a test to a Gmail account you control. Open the message, choose "Show original", and check SPF, DKIM, and DMARC all say
PASS. - Check whether it is one provider or all of them. One provider is a reputation problem with that provider; all of them is an authentication problem.
- Check your bounce and complaint rates for a recent spike.
- Check you have exactly one SPF record — two is a permanent failure.
Recovering a damaged reputation
There is no fast fix. Stop sending marketing entirely, keep transactional mail flowing, fix whatever caused the complaints, then warm up again over several weeks. Switching to a new domain to escape a bad reputation works briefly and then fails, because the behaviour that caused the problem follows you.
Related