SMTP Error 421 : The Mail transfer service is unavailable because of a transient event. SMTP reply 451 can be caused by many things but generally indicates that the mail server which returns this status code is currently unavailable but may be available later.
For example, the server administrator may have stopped the mail service to troubleshoot a problem, or the mail server is right in the middle of rebooting, or the mail server is currently processing too many incoming messages or incoming requests, etc…. Note : “Mail Server” in this case can be any of the mail servers on the message’s route – the sending server (your server), the ISP SMTP server, or the recipient’s mail server. Clearly, if you repeatedly receive an SMTP status 421 then the problem is no longer of a transient nature and you need to investigate or inform the relevant network administrator, ISP tech support, or the recipient.

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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. 220 – <Server Name> service is running (also called SMTP Error 2.2.0) SMTP Status 220 : This is normally the first message...
3. 251 – User not local will forward (also called SMTP Error 2.5.1) SMTP Status 251 : The email account is not local...
4. 221 – The domain service is closing the transmission channel (also called SMTP Error 2.2.1) SMTP Error 221 : The server is ending the mail...
5. 432 – The recipient’s Exchange Server incoming mail queue has been stopped (also called SMTP Error 4.3.2) SMTP Error 432 : This is an SMTP status response...

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