SMTP Error 451 : The action has been aborted by the ISP’s server. “Local” refers to the ISP’s server. This error is usually due to overloading at the ISP from too many messages or transient failures. Typically some [hopefully] temporary event prevents the successful sending of the message. The next attempt to send by your server may prove successful.
If this error keeps occurring to the point that it has effectively lost its transient nature and has become ….. frequent (!!), then the problem is at your end and you should check your own mail server (if you email out of a corporate network), communications on your side (router, server network card), or inform your ISP if your mail server relays through your ISP or if you are a home user emailing out through Outlook, Outlook Express, Windows Mail, or similar email program.

Example of typical SMTP Error 451 return messages :
“SMTP error 451 Unable to complete command, DNS not available or timed out” or “451 Domain of sender address does not resolve” or “451 Error getting LDAP results in map”.

Related posts:

1. 450 – Requested action not taken – The mailbox was unavailable at the remote end. A secondary SMTP error code may follow “450” to refine the reason for the failure to transmit the message, e.g. “SMTP Error 450 (also called SMTP Error 4.5.0) SMTP Error 450 : The server could not access the...
2. 452 – Requested action not taken – Insufficient storage. (also called SMTP Error 4.5.2) SMTP Error 452 : The ISP server’s disk system has...
3. 250 – Requested mail action OK completed (also called SMTP Error 2.5.0) SMTP Status 250 : The mail server has successfully delivered...
4. 251 – User not local will forward (also called SMTP Error 2.5.1) SMTP Status 251 : The email account is not local...
5. 421 – <Server name> Service not available – the sending email program should try again later (also called SMTP Error 4.2.1) SMTP Error 421 : The Mail transfer service is unavailable...

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