How to detect fake bounces from spam filters?
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A real bounce means the address doesn't exist or the mailbox is full. A fake bounce, sometimes called a false bounce, is a rejection signal that comes from a spam filter, not from a missing address. The difference matters because treating fake bounces as real ones can cause you to suppress addresses that would have received your email from other senders.
The patterns that suggest you're looking at a fake bounce:
Recent successful delivery, then sudden bounce. If you sent to this address last week without issue and it's bouncing today with no other obvious change, the address probably still exists. Something about your current send triggered filtering.
Domain-level pattern. If every address at company.com starts bouncing simultaneously but individual addresses at the same domain have worked before, it's more likely a domain-level block or aggressive filter than a mass account deletion.
Non-standard SMTP codes. Standard hard bounces use 5.1.1 or 5.1.2 (user not found, domain not found). Spam filter rejections often produce unusual codes, 5.7.x range, with message text that mentions policy rather than invalid addresses.
Works from some senders, not others. If you can verify the address sends and receives from other sources, the bounce is about your domain's reputation with that receiver, not the address validity.
The diagnosis tool: read the full SMTP bounce response, not just the summary your ESP shows. The detailed message usually tells you whether it's a policy rejection or a genuine address error. You can also check your domain and IP against common blocklists. If you're listed, that explains filter-level rejections. Our free blocklist checker covers most major lists.
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