What is a “non-standard bounce” (custom message)?
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You send an email, it bounces, and the error message your system gets back looks nothing like the standard codes you'd expect. No clean 550 or 421. Instead, something like "RP-001" or a paragraph of policy text. That's a non-standard bounce.
Standard bounce messages follow RFC specifications for SMTP responses. They use defined numeric codes (like 550 for a permanent failure or 421 for a temporary one) so any receiving system can interpret them automatically. Non-standard bounces skip that structure. They use custom codes, proprietary reason strings, or plain-text messages that don't map to anything in the RFC rulebook.
Microsoft 365 is a good example. It returns custom codes like "RP-001" for rate limiting situations that don't have a direct RFC equivalent. Gmail goes the other direction entirely, sending back detailed policy descriptions in plain English instead of numeric codes. Legacy mail systems sometimes return completely unique error strings that only make sense in the context of that specific setup.
The challenge for bounce processors is that these messages can't be parsed the same way. Instead of reading a code, the processor has to scan the message text for keywords ("quota exceeded", "policy violation", "rate limit") and make a judgment call about what category it fits. That works reasonably well, but it means the same underlying problem can get classified differently depending on how the provider worded it that day. And providers don't announce when they change their error messages.
If you're managing your own bounce processing, the practical approach is to build provider-specific parsing rules for the mailbox providers you send to most, fall back to conservative classification for anything you don't recognize, and flag unknown patterns for review so you can update your rules over time. Suppressing too aggressively on unrecognised bounces can hurt your deliverability. Ignoring them entirely can hurt it worse.
If you're not sure how your current setup is handling these, our SOS hotline is free and we're happy to take a look with you.
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