Bad Envelope FROM when using alias from different domain.

Hello,

I found a error in Pantheon Mail which breaks DMARC checks.

When I have account with alias from different domain created this way:
username: me@example.com
e-mail address: me@example.com
e-mail alias: me@mydomain.com

**Pantheon mail is using wrong envelope-from**, if I use one of aliases.

If I select to use mail me@example.com in new-mail dialog, it uses these values in headers:
Envelope-From: me@example.com
Body From: me@example.com

When I select me@mydomain.com as sender, then it uses these values:
Envelope-From: me@example.com
Body From: me@mydomain.com

In second case it can break DMARC verification check on recipient's side and e-mail will be quarantined/rejected (depends on policy for mydomain.com).

Envelope-From: and From: header should be aligned (from same domain).

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