Mail on iPhone sometimes uses wrong outgoing mail account

I have selected my .me email account as the default for outgoing email messages. But my iPhone does not always use that account. When I hit the "compose" icon a new blank email message opens up and my .me account always appears in the "From:" space. HOWEVER, when I then enter the recipient's email address my iPhone sometimes, but not always, changes the "From" account to my Gmail account.

There are certain recipients for whom it always uses the correct default outgoing address (my .me account), and there are other recipients for whoom it always changes the outgoing account, but I haven't been able to identify a common thread that would tell me why it is making the change for those recipients.

I have an iPad, which is set to use the same default outgoing email account, and have not had this problem on that device. Nor on my iMac.