SMTP Error 450 : The server could not access the mailbox to deliver the message. This could be caused by a process on the remote server tidying up the mailbox, or the remote mailbox could be corrupt, or the remote mailbox may be stored on another server which is currently offline, or the network connection went down while sending, or the remote mail server does not want to accept mail from your server for some reason (IP address, blacklisting, etc..). In general SMTP Error 450 is a transient error at the remote end (the destination), or at one of the servers en route to the remote end, and should induce your mail server to retry after it’s preset retry interval.

Example of an SMTP Error 450 reply message : “450 Please try again later”.

SMTP Error 450 is often followed by a second SMTP error code to refine the reason for the email not reaching its destination. For example : “SMTP Error 450 5.2.3 Msg Size greater than allowed by Remote Host”. When that is the case and If the error message is not as clearly worded as in this example, then simply search this document for the secondary error code. In this case searching this document for SMTP Error 523 or SMTP Error 5.2.3 would yield an explanation identical to the wording above.

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