Message-ID header always uses the domain from the account's first address

Bug Summary

After setting up a single account that can send from multiple domains, outgoing Message-ID headers don't match the From: domain of the email. Instead, it's always the domain of the first-listed address for the account.

Could be a privacy issue if the user wants to keep their sending identities separate (but uses a single mail account, like I do).

Your installation

• Geary version: 3.38.0
• Installation method: deb (via Pop! Shop)
• Desktop environment: pop:GNOME
• Operating system and version: Pop!_OS 20.10
• Email provider: fastmail.com

Steps to reproduce

1. Create a single account
2. Add email address: foo@foo.com
3. Add email address: bar@bar.net
4. Send an email as foo@foo.com
5. Send an email as bar@bar.net

What happened?

Both of the received emails will have a Message-ID header ending in @foo.com. (If bar@bar.net is moved to the top, it will always be used, instead.)

What did you expect to happen?

Message-ID should match the sending address.