ERROR : “Remote host said: 550 Requested action not taken: mailbox unavailable Giving up on
REASON : If sending address is invalid,then you will get this message
ERROR : Sorry, I couldn’t find any host named
REASON : Problem with the recipient domain. i.e , The domain name specified is unknown. Either you typed it incorrectly or that domain no longer exists.
ERROR : “Remote host said: 550 abc [at] example [dot] com: Recipient address rejected:" unknown user
REASON : The email address specified by you was incorrect. kindly check the recipient email address.
ERROR : Remote host said: 554 Message type not allowed. UP Email not accepted for policy reasons
REASON : Please add SPF records for your DNS and send the mail.