I've raised a similar issue before, but we didn't fully have the email service configured.
We've now got that configured, and I've set it up so it forwards failures to myself.
Every user that tries to respond via email, an email goes to me with the following error;
The error was:
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"User '<user>' does not have permission to comment on an issue in project '10014'.
"
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I'm able to respond to tickets via email myself.
But I have a license to SeriviceDesk as I'm an operative.
It's almost as though it's suggesting they need a license to comment on an issue???