Most internet and email service providers will customize their outgoing bounce messages to conform to their network's terms of service, however SMTP email has a standard set of codes in use across the entire internet, globally.
Suppose you attempt to send a message to an address that does not exist.
ISP #1 may send a message of
Remote-MTA: dns; smtp.store.example [192.0.2.3] Diagnostic-Code: smtp; 550 No such user hereWhile ISP #2 may send a message of
while talking to smtp.store.example [192.0.2.3] >>> RCPT TO:<firstname.lastname@example.org> <<< 550 No such user hereThere are many SMTP bounce codes. The vast majority of mainstream internet and email service providers maintain a list of their codes and messages with meanings. Below are some links to a random sampling of some ISPs bounce codes and messages.
- America Online (AOL) Postmaster Error codes
- Google Gmail - SMTP Error Reference
- Microsoft Postmaster Codes (This includes Hotmail.com, Outlook.com, Live.com and msn.com)
- Comcast SMTP Error Codes
- AT&T Error List (This includes all AT&T domains found here)
- Yahoo Commonly Searched Postmaster Errors
- Time Warner\Road Runner Common Error Messages
If you would like to know what a soft bounce is, please What is a soft undeliverable email (soft bounce)?.
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