Are the users' gmail accounts 'gmail.com' or 'mydomain'? Because I've sent mail from servers using a google mailbox to authenticate, and the from field was overwritten with the name of the mailbox used.
The users' gmail accounts are at user@gmail.com, but they're sending mail as user@mydomain, via mydomain's authenticated SMTP server. This is exactly the scenario described in the comment at jwz.org.
I read it as the opposite of jwz. You say your users are sending mail via "mydomain" SMTP. jwz says his users are sending mail "from their phone up to Google's SMTP server".
The goog then sends mail to the receiver at 'dnalounge' MX, where it's then shuffled back to the receiver's gmail mailbox. I don't read his setup as having the sender involved with the dnalounge SMTP server at all.
