I got this when sending using SmarterMail but using Yahoo Mail, it's delivered successfully and even got a response from the recipient. What could be the issue?
Original-Recipient: rfc822Failed Recipient: firstname.lastname@example.org
Final-Recipient: rfc822Failed Recipient: email@example.com
Diagnostic-Code: smtpCould not deliver message to the following recipient(s):
Failed Recipient: firstname.lastname@example.org
Reason: Remote host said: 550 Requested action not taken: mailbox unavailable
invalid DNS MX or A/AAAA resource record