Providers such as Gmail and Outlook do not publish their formulas, but the inputs are well known: spam complaints, bounces, spam trap hits, how many people open, reply or delete without reading, whether you authenticate, and how steady your volume is. Domain reputation follows your domain wherever you send from; IP reputation belongs to the server, which on Google or Microsoft mailboxes you share with many senders.
What hurts it
- Spam complaints. Gmail asks senders to keep their reported spam rate below 0.3%, and lower is better.
- High bounce rates from unverified lists.
- Hitting spam traps, which signals a bought or scraped list.
- Sudden volume jumps from a mailbox with little history.
A new domain sends 400 cold emails on day two. 30 bounce, 6 people mark it as spam. The next day, even warmup messages start landing in spam.
How to protect it
Send cold email from separate domains so your main domain stays clean, authenticate every one with SPF, DKIM and DMARC, warm up every new mailbox, verify lists, and stop sending from a mailbox at the first sign of trouble. Google Postmaster Tools shows your domain reputation once you send enough mail to Gmail.
ZicReach watches this per mailbox with a health score: 100, minus one point for each warmup email that landed in spam in the last 14 days, minus a bounce penalty. Below your limit, 90 by default, campaigns pause on that mailbox while warmup continues. See it on the warmup and deliverability page.