I know this is a very old question, but I didn’t see anyone else answer it and I think I have the solution. At least it’s working for me. I don’t know if there’s any unintended consequences of doing this because I’m just getting started, but here you go.
I liked the idea of categories vs groups so I was following this setup guide, but I also wanted to have an open forum and I ran into the same issue stated above.
If you do a search for email settings in in admin you’ll find the page below. It shows a trust level of 2 needed for users to create posts via email.
Staged users bypass that requirement, but once they become full users they’re still trust level 0 and e-mail breaks. So just remove TL2 and add TL0 and you’re golden.
I did this along with using the Private Topics Plugin linked above so users can request support directly on the website too and I’m almost done.
I just need to neatly integrate priority and status tags and I think we’ll be set.
