We are suddenly unable to reply to tickets, every time I reply to a ticket I get an "Email was not delivered to ___" error message with the following explanation:
The following message composed by you on ticket 903 failed to reach the recipient EMAILADDRESS@DOMAIN.com.
Reason: "Message bounced - Temporary error.
This has been working fine for a over a year, and just started failing on Wednesday, 5/22/19.