Contact AT&T mail support and see if they can recover the messages.
From what is posted over in the AT&T forums, it does not appear that AT&T provides deleted-message recovery.
https://forums.att.com/conversations/data-messaging-features-internet-tethering/can-i-recover-deleted-messages/5fb80abf97f6d57f4e8d0172?commentId=5fb80c3197f6d57f4e8d017f
If there are iCloud or local backups to a Mac or Windows system, the backup that was created most recently prior to the deletion might have the mail messages, though I'm doubtful of that, and though that also means replacing your entire iPhone contents with the backup. Everything on your iPhone that was changed after the to-be-restored-backup was created will revert to the contents of that backup.
If you do not have iCloud or local backups of your iPhone, (or backups of another device or Mac or Windows system configured to share that same AT&T email account) there's nothing to restore.
Or you ask the sender to re-send the messages.