What does 500 mean?
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If you've seen a 500 error in your mail logs, the receiving server is essentially saying: "I didn't understand what you just sent me." It's a syntax error at the SMTP protocol level, meaning the command your sending server issued was broken, unrecognized, or sent in the wrong order.
This is a permanent 5xx error, so the receiving server won't retry. The message won't get through until whatever caused it is fixed.
Common error messages you might see include things like 500 Syntax error, command unrecognized, 500 Unrecognized command, or 500 Error: bad syntax.
The most common causes are a misconfigured mail server, a software bug in the mail client or sending platform, extended SMTP commands that the receiving server doesn't support, or character encoding problems in the command itself.
The good news is that a 500 almost always points back to the sender's side (your setup or your ESP's), not the recipient's server. So if you're seeing this in logs, start there. Check whether your mail server configuration follows standard SMTP formatting. If you're using a third-party ESP and the error is persistent, their support team should be able to read the raw logs and pinpoint exactly where the malformed command is coming from.
Not sure how to read your email headers or find where the error is happening? Our free Email Header Analyzer can help you trace what's going on, or reach out via our SOS hotline if you're stuck.
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