Agree with GreenJim. I’m willing to bet you’re using Office365.
Assuming you are using Postfix, to assist with debug, try creating/editing ‘/etc/postfix/header_checks’.
Add the following to the header_checks file: -
You’ll have to make postfix aware of the header checks file in /etc/postfix/main.cf by adding the following line: -
header_checks = regexp:/etc/postfix/header_checks
Restart postfix and look at your mail log. You should see mail envelope details in there when a mail is sent. Hopefully this will let you see what’s going wrong.
If the mail envelope headers mismatch in any way, or from/sender envelopes don’t match the email address actually used by RT to log into Office365, it will return this message.
You say you are only seeing the issue when someone replied to a ticket, which is when RT will try to send comments to your queue watchers.
Perhaps the email is ‘sent from’ your main queue email address, however it’s from/sender envelopes are for the comment address?
As GreenJim alluded to, try writing your own template with hardcoded headers.
For example, make a new template based (on a global one), and add the following text to the start of the template (a blank line is required after it) : -
The above is obviously just an example - don’t expect randomly changing envelope headers will work. You’ll need to look very carefully at the mail log for the envelope headers when it fails, and try to correct in the template.