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”.

SMTP Status 251 : The email account is not local to the ISP server but the ISP server will accept the email and will forward it (the server will RELAY your message, this is the most common action for ISP Mail servers – the recipient will see your ISP in the mail header as one of the first hops on the way to the recipient’s email system). SMTP Error 251 is therefore more of an informational message for technicians tracking how a message reached its destination.