MyBB Community Forums

The admin address that I'm using in my board (Site Details) is a @mysite.org kind of email, which forwards all incoming messages to my main @gmail.com mailbox.

What do I have to do if I want to reply to members using my @gmail.com email but only letting them see the @mysite.org one in replies?

You can specify a different email address for the contact form. The setting is named "Contact Email".