> If there were SMTP response codes in the 5xx range, that's a perm failure, end of story. This is, plain and simply, wrong. The RFC does not state that. RFC

> If there were SMTP response codes in the 5xx range, that's a perm failure, end of story. This is, plain and simply, wrong. The RFC does not state that. RFC 5321 4.2.1 Reply Code Severities and Theory > 5yz Permanent Negative Completion reply > The command was not accepted and the requested action did not occur. The SMTP client SHOULD NOT repeat the exact request (in the same sequence). Even some "permanent" error conditions can be corrected, so the human user may want to direct the SMTP client to reinitiate the command sequence by direct action at some point in the future (e.g., after the spelling has been changed, or the user has altered the account status) SHOULD NOT is also clearly defined by RFC 2119: > This phrase, or the phrase "NOT RECOMMENDED" mean that there may exist valid reasons in particular circumstances when the particular behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior described with this label.