When email stops reaching the inbox, the instinct is to rewrite the copy. It is the most visible part of the campaign and the easiest thing to change. It is also, in most cases, not the problem.
This is an ordered checklist. Work through it in sequence, because the steps are deliberately arranged from most to least likely.
Step 1 — Authentication (15 minutes)
Fast, unambiguous, and a hard requirement for bulk senders. Run your sending domain through an authentication check and confirm three things:
- SPF present, a single record, and under the ten-lookup limit.
- DKIM signing, with the signing domain matching your From domain.
- DMARC published, and passing with alignment.
The subtle failure is alignment: SPF can pass while DMARC fails, because SPF validated a platform's domain rather than yours. Our free checker reports all three, and the full guide explains what each is asserting.
Also open the raw headers of a message you have actually sent and read the Authentication-Results line. That is what the receiver concluded, which is the only opinion that counts.
Step 2 — Reputation (30 minutes)
Set up Google Postmaster Tools if you have not. It shows domain reputation, IP reputation, spam complaint rate and authentication success for your Gmail traffic — close to a direct read on your standing with the provider that probably holds most of your list.
Look for:
- Domain reputation at Low or Bad. Anything below High needs attention.
- Spam rate approaching or above 0.3%. The recommendation is under 0.1%.
- Authentication success below 100% — meaning some of your mail is failing.
Then check blocklists for both your domain and sending IP. Weight the result by which list — a Spamhaus listing matters, an obscure one usually does not. Our blocklist guide covers delisting.
Step 3 — List quality (1 hour)
Pull the numbers for your last several campaigns:
| Metric | Healthy | Investigate |
|---|---|---|
| Hard bounce rate | Under 1% | Above 2% |
| Total bounce rate | Under 2% | Above 5% |
| Complaint rate | Under 0.1% | Above 0.3% |
| Unsubscribe rate | Under 0.5% | Rising over time |
Then ask the harder question: what proportion of your list has engaged in the last 90 days? If most of it has not, that is very likely your answer. Engagement is measured as a ratio, so unengaged contacts actively suppress placement for the engaged ones.
Also account for where the list came from. Any purchased or scraped data explains the problem on its own and cannot be fixed by anything else on this checklist.
Step 4 — Sending patterns (30 minutes)
Three questions:
- How much is each mailbox sending? Concentrating volume on one address is the most detectable pattern in bulk email. Ceilings are covered in our sending limits guide.
- How fast? Four hundred messages in ten minutes and four hundred across a working day are the same volume and entirely different signals.
- Has volume changed abruptly? A jump from 500 a day to 15,000 without a ramp looks like a compromised account.
Check for temporary rejections — 421 and 429 responses — in your sending logs. Their presence means providers are already throttling you, which is a signal that precedes filtering.
Step 5 — Content (last, and briefly)
Now look at the copy. Check for excessive capitals, high link density, missing unsubscribe wording, and the trigger phrases our spam score checker flags. Confirm every campaign carries a working one-click unsubscribe.
Fix what is obviously wrong, then stop. If steps one to four were clean and the problem persists, content is worth deeper investigation. If they were not, content changes will not rescue you.
Reading the pattern
Two diagnostic shortcuts worth knowing.
If every campaign dropped at once, including ones that previously performed well, it is reputation or authentication. Content does not cause a uniform collapse.
If the problem is at one provider only, it is a reputation issue specific to that provider. Configuration faults tend to fail everywhere simultaneously, so a Gmail-only problem points at your Gmail standing rather than your DNS.
The recovery sequence
Once you have found the cause, recovery follows a consistent shape: stop the behaviour, send only to your most engaged segment for two to four weeks, fix the technical baseline, clean the list thoroughly, then reintroduce volume gradually. It is covered in more detail in our guide to sender reputation.
Set a calendar reminder to repeat this audit quarterly. Deliverability degrades quietly, and the point of a checklist is to notice before a campaign tells you.