Alice lives in NYC and Bob lives in Vancouver Using SMTP Ali

Alice lives in NYC and Bob lives in Vancouver Using SMTP, Alice attempts to send an email from her mail server to Bob\'s mail server, unfortunately, Bob\'s mail server is down. Given this failure, does Alice\'s email get stored at some intermediate mail server (neither on Alice\'s nor on Bob\'s mail servers)?

Solution

Yes, the mail is stored in an intermediate mail server and usually kept for 48 hours and in this time span re attempt of sending the mail again is performed.

Generally SMTP server sends error 451.

This response is sent when the message simply failed. Often times this is not caused by you, but rather because of a far-end server problem.