Where you saw it: Recipients telling you they found it in junk, open rates collapsing on one provider, or a seed test showing inbox placement at one mailbox provider and spam at another.
Why it happens
- Authentication is incomplete. Without SPF, DKIM and DMARC, you look like an unverified sender, and bulk senders to Gmail and Yahoo have needed all three since February 2024.
- Complaint rate above 0.3%, which is the threshold Google and Yahoo both name. A handful of spam-button presses per thousand messages is enough.
- You are mailing addresses that have never engaged. Gmail in particular weighs recipient behavior heavily, and a list of silent addresses teaches it to file you away.
- A shared IP carrying another sender's reputation problem, which is invisible from your side of it.
- Content signals: a URL shortener, a domain with no history, an image-only message, or a link to a site on a blocklist.
- Volume that jumped suddenly. Going from 500 messages a day to 50,000 without warming up reads as a compromised account.
The fix, in order
Fix authentication before anything else
Publish SPF, DKIM and DMARC, and confirm all three pass with alignment on a real message. Nothing else on this list matters while this is broken.
Open Google Postmaster Tools and Microsoft SNDS
These show your complaint rate and reputation from the receiver's own perspective. Guessing at reputation when both providers will tell you is wasted effort.
Cut the list to people who have engaged
Send only to addresses that opened or clicked in the last ninety days for two or three sends. Volume drops, engagement rate climbs, and the reputation signal reverses.
Make unsubscribing trivial
Add the List-Unsubscribe header with one-click support. A person who unsubscribes costs you nothing. A person who presses the spam button costs you reputation with every other recipient at that provider.
Warm up before you scale
Raise volume gradually over two to four weeks on a new domain or IP, sending to your most engaged addresses first.
Verify the list to remove bounces
This removes invalid addresses and the reputation damage they cause. It does not fix authentication, content or complaints, and it is worth being clear that those are separate problems.
How to know it worked
The short version
- Postmaster Tools shows domain reputation at high or medium and complaint rate below 0.1%.
- A seed test across Gmail, Outlook, Yahoo and a corporate domain shows inbox placement.
- Open rates recover on the provider where they fell, which is the measure that matters.
Questions people ask
Will cleaning my list stop mail going to spam?
It removes one cause. A high bounce rate does damage reputation, so fixing it helps. If your authentication is broken or your complaint rate is high, a clean list changes nothing on its own.
How long does reputation take to recover?
Weeks of consistent, engaged sending. There is no reset button. Providers rebuild trust from behavior over time, and a single good send does not move it.
Why does Gmail filter me and Outlook does not?
They weigh different signals. Gmail leans heavily on recipient engagement, Microsoft leans more on complaint rate and sending IP history. It is normal to have a problem at one and not the other.