Where you saw it: A Gmail rejection citing the sender guidelines, a warning in Postmaster Tools, or a sudden drop in Gmail delivery after February 2024.
Why it happens
- Missing or failing DMARC on the sending domain, which is the requirement most senders had not met.
- No List-Unsubscribe header with one-click support, which is mandatory rather than recommended for bulk senders.
- Spam complaint rate above 0.3%, measured in Postmaster Tools.
- The From domain not aligning with the authenticated domain.
- Missing valid forward and reverse DNS on the sending IP.
The fix, in order
Publish and pass all three authentication mechanisms
SPF, DKIM and DMARC, with at least one of SPF and DKIM aligned to your From domain. Confirm on a real message rather than assuming the platform handled it.
Add one-click unsubscribe properly
Include both List-Unsubscribe and List-Unsubscribe-Post headers so Gmail can show its own unsubscribe control. A link in the footer alone does not satisfy the requirement.
Honor unsubscribes within two days
Google's guidelines specify processing within 48 hours. A weekly export from your platform does not meet that.
Get complaint rate under 0.1%
The stated ceiling is 0.3% and the target is below 0.1%. Watch it in Postmaster Tools, which is the only place the real number appears.
Fix reverse DNS on the sending IP
Valid forward and reverse DNS is part of the requirements and is often the thing nobody checked.
Clean the list to protect the rate
High bounce rates worsen reputation, which feeds the complaint problem. Verification addresses that one input and does nothing about the other five items here.
How to know it worked
The short version
- Postmaster Tools shows all authentication passing and complaint rate below 0.1%.
- A test message in Gmail shows the unsubscribe control next to the sender name.
- Domain reputation reads high or medium rather than low or bad.
Questions people ask
What counts as 5,000 messages a day?
Messages to Gmail addresses from the same domain, counted across everything you send. Once you cross it once, Google treats you as a bulk sender from then on.
What happens if I do not comply?
Rejection or junk filing of Gmail traffic. Google phased enforcement in during 2024 and it is now the normal state rather than a warning period.
Does the 0.3% threshold include unsubscribes?
No. It counts spam button presses only. That is precisely why making unsubscribe easy protects you: an unsubscribe costs you nothing against this number.