421 Service not available, closing transmission channel
450 Requested mail action not taken: mailbox unavailable (e.g., mailbox busy)
451 Requested action aborted: error in processing
550 User’s mailbox was unavailable (such as not found)
551 The recipient is not local to the server.
553 The command was aborted because the mailbox name is invalid.
5.0.0 Address does not exist
5.1.1 Bad destination mailbox address
5.1.2 Bad destination system address
5.1.3 Bad destination mailbox address syntax
5.1.4 Destination mailbox address ambiguous
5.1.5 Destination mailbox address valid
5.1.6 Mailbox has moved
5.1.7 Bad sender’s mailbox address syntax
5.1.8 Bad sender’s system address
5.2.0 Other or undefined mailbox status
5.2.1 Mailbox disabled, not accepting messages
5.1.0 Other address status
5.3.0 Other or undefined mail system status
5.4.1 No answer from host
5.4.2 Bad connection
5.4.0 Other or undefined network or routing status
5.4.3 Routing server failure
5.4.4 Unable to route
5.4.7 Delivery time expired
5.5.0 Other or undefined protocol status