Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)
The destination mail server was unable to verify the email address used. This is most commonly a red flag that it's not a valid email address, however may also mean that the receiving mail server has firewall or connectivity issues.
Check with the recipient to ensure the email address is correct.
550 Invalid recipient
550 User account is unavailable
550 No such user here