Yeah, it’s the FAS hookup thing. I just looked and as an admin I can edit your username and full name, but the email comes from the linked account. It says “Never shown to the public”, for what that’s worth.
For what it’s worth, I’m also not sure of the FAS implications of changing the username. Leaving all accounts matching FAS is kind of nice from a metrics POV – not that I’d do individual tracking, but for counting total active Fedora folks.
Oh, hmmm… there’s a “secondary email” thing. Let me see if that can be activated along with external auth. Stand by. 
What mail client do you use? I know mutt has the ability to change outgoing email address based on the recipient, and maybe yours has something similar? As a fallback if I can’t figure out the secondary email thing.