When using the Defender OTP Email tokens, the tokens are not being sent. When reviewing the Defender logs, a warning such as this is returned:
"Fri 06 Oct 2017 18:14:33 Defender E-mail OTP: Failed to send 0x80004005
Fri 06 Oct 2017 18:14:33 Defender E-mail OTP: Error : Unexpected error sending an e-mail.
Fri 06 Oct 2017 18:14:33 Defender E-mail OTP: Exception : System.Net.Mail.SmtpFailedRecipientException: Mailbox unavailable. The server response was: 5.7.1 <first.last@domain.com>... Relaying denied"