Email deliverability

Inbox providers such as Gmail and Outlook score every message using the sending domain's and IP address's history, authentication (SPF, DKIM, DMARC), how recipients react (opens, replies, spam reports) and the content itself. A high bounce rate from old or mistyped addresses, or spam complaints above about 0.3% (the limit Gmail publishes), quickly pushes mail into spam.

Habits that help: send only to people who asked for it, confirm sign-ups, remove bounced addresses automatically, keep marketing on a separate subdomain or stream from password resets, warm up a new domain or dedicated IP by raising volume gradually, and put a working unsubscribe link in anything promotional. Google Postmaster Tools shows how Gmail sees your domain.