What is “poor sender reputation” in bounce messages?

You sent a campaign, and instead of delivery confirmations you got a wall of bounces saying things like "Rejected due to poor sender reputation" or "Sender IP has poor reputation." That's an ISP telling you, pretty directly, that your sending history has left a bad impression.

Sender reputation is a score (or set of signals) that mailbox providers build up about your domain and IP over time. When that score drops below their threshold, they don't just filter your email. They reject it outright, and the bounce message tells you why.

Several things feed into a poor reputation score. High complaint rates are the biggest one. When too many recipients mark your mail as spam, that's a direct vote against you. Sending to invalid addresses (high bounce rates), hitting spam traps, and having low engagement all pile on top of that. Past blocklist events also leave a mark, even after the listing is cleared.

The bounce messages themselves vary by provider, but you'll typically see phrases like "Sender IP has poor reputation," "Domain reputation too low," or just a generic spam rejection with a reference to a reputation tool like Spamhaus or Talos. The exact wording matters less than the pattern. If you're seeing reputation-based rejections from multiple providers at once, the problem is your sending history, not a one-off policy issue.

Fixing it takes time (of course, that's the frustrating part). There's no single switch to flip. The path forward usually looks like this:

• Audit your list. Remove addresses you've never had real engagement from, especially anything you haven't mailed in over six months.
• Drop your sending volume temporarily. Rebuilding reputation works better at lower volume with higher engagement than blasting a full list and hoping for the best.
• Check your authentication setup. SPF, DKIM, and DMARC don't fix reputation on their own, but sending without them makes everything worse.
• Watch your postmaster tools. Gmail and Outlook both have free postmaster dashboards that show you how your reputation is trending.

Think of sender reputation like a credit score with ISPs. You built it up slowly with good behavior, and you can rebuild it the same way. It just won't happen overnight.

If you want to check whether your domain is already on a blocklist, our free blocklist checker takes about 30 seconds. And if things feel genuinely stuck, our SOS hotline is free to use.