It would help a lot if you wrote what you were actually using for settings, even anonymised slightly.
You might also want to look into setting alternative_reply_by_email_addresses.
I don’t believe so.
If possible, for your setup I’d recommend something like:
- set
reply_by_email_addresstoinbound+%{reply_key}@forum.hostname - configure a mail server to receive email for
forum.hostnameand deliver it all tothe.gmail.account@gmail.com
Here the intermediate mail server doesn’t need to understand plus-addressing, it just needs to forward on everything for the domain.