What is “poor sender reputation” in bounce messages?

"Poor sender reputation" signals accumulated negative history:

What contributes to poor reputation:

Complaint rates: Recipients marking mail as spam. Bounce rates: Sending to invalid addresses. Spamtrap hits: Messages to monitoring addresses. Engagement: Low opens, clicks; high ignores. Blocklist history: Past listing events.

How it appears in bounces:

"Rejected due to poor sender reputation." "Sender IP has poor reputation." "Domain reputation too low." Generic "spam" rejection with reputation tools referenced.

Resolution path:

Audit list quality and sources. Remove unengaged subscribers. Improve authentication. Reduce volume while rebuilding. Monitor improvement via postmaster tools.

Reputation is your accumulated credit history with ISPs. Poor scores take time and consistent good behavior to improve.