How do mailbox providers score IPs?

Mailbox providers maintain sophisticated scoring systems that evaluate IP reputation continuously. The key factors:

Complaint rates: The percentage of recipients who mark your mail as spam. This is the strongest negative signal. Rates above 0.1% raise flags; above 0.3% causes serious problems.

Bounce rates: High rates of invalid addresses suggest poor list hygiene or purchased lists. Hard bounces (invalid addresses) hurt more than soft bounces (temporary issues).

Spamtrap hits: Sending to spamtrap addresses indicates list problems. Pristine traps (never valid) suggest purchased lists; recycled traps (abandoned addresses) indicate poor hygiene.

Engagement signals: Gmail in particular weighs recipient behavior. Opens, clicks, replies, and moving messages from spam to inbox improve reputation. Deleting without reading or ignoring messages hurts.

Volume patterns: Sudden spikes raise suspicion. Consistent, predictable sending establishes trust. New IPs face extra scrutiny until patterns establish.

Authentication: Proper SPF, DKIM, and DMARC alignment contribute positively. Failed authentication triggers filtering.

Each provider weighs these factors differently. Google emphasizes engagement; Microsoft weighs complaints heavily. There's no universal formula, which is why monitoring tools like Google Postmaster Tools and Microsoft SNDS are essential.