IPhone is overriding the Account Email with a different address

I'm looking into an issue on an iPhone which has me stumped. It has a number of Accounts configured:

• iCloud (myname@icloud.com)
• Exchange (myname@mycompany.com)
• Gmail (myname@gmail.com)
• Outlook (myname@hotmail.com)

iCloud and Exchange seem to work fine. But when we add the Outlook account (myname@hotmail.com), it is configured successfully, until it is saved, then the Email field in Account changes to show the Gmail address (myname@gmail.com) and will not change back. The Mail app also shows two entries with the Gmail address when I try to switch the "from" account.

When I send an email from within the Outlook account within the Mail app, it displays the Gmail address (myname@gmail.com) as the "from" address, and replies go to Gmail.

I've tried:

• Deleting the Gmail account
• Deleting the Outlook account
• Re-adding the Outlook account (still has the same problem, even if the Gmail account is not present on the phone)
• Re-adding the Outlook account as an Exchange account
• Factory reset the phone and restore from backup
• Factory reset the phone and start fresh
• Get a new phone

None of these steps have solved the problem, the phantom Gmail account (myname@gmail.com) always overrides the Outlook one (myname@hotmail.com), even if the Gmail account is not configured on the phone.

Interestingly, none of the other accounts have this issue. I've also added a different Outlook account (myname2@hotmail.com) and this does not have the same problem either.

It appears to be something to do with adding an account called myname@hotmail.com but I'm absolutely stumped.

Has anyone encountered this issue, or can someone suggest some more troubleshooting steps please?