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When you get a message from a "MAILER-DAEMON" or a "Mail Delivery Subsystem" with a subject similar to "Failed Delivery," this means that an email you sent was undeliverable and has been bounced back to you.
554 Transaction has failed (Or, in the case of a connection-opening response, "No SMTP service here") 554 5.3.4 Message too big for system [ 4 ] 556 Domain does not accept mail [ 5 ]
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A relay server typically determines which server to connect to by looking up the MX (Mail eXchange) DNS resource record for each recipient's domain name. If no MX record is found, a conformant relaying server (not all are) instead looks up the A record. Relay servers can also be configured to use a smart host.