The user has two email accounts, which I'll call firstname.lastname@example.org - personal, and email@example.com
The company account is hosted by Google.
Just a few days ago, for unknown reasons, all email from their personal account started being sent from the company account instead.
They called Apple, which tried a few things, and then said it was a Google problem.
I had them delete all their email accounts from their iPhone, power off the iPhone, and then set up their personal account on the iPhone again.
They can receive email sent to their personal account, but sent email is still being sent from the company account.
I didn't yet have them "Reset All Settings" on the iPhone.
What might have caused this, and how might it be corrected?