iMessage insisting on texting to wrong email address

Hello - for some reason, for one of my contacts, my iPhone keeps defaulting to an old email address when I send an iMessage. I've deleted this contact's email address and when I go to text them, my phone insists on using that old email address, and not their phone number, or new email address. The only way I can message this contact and not revert to SMS, is to physically open their contact information and iMessage them from there. When I go through Messages, it reverts to the deleted email address, and identifies them by the old email as well, and only wants send via SMS.

I've deleted the contact and re-entered it (without the old email address) and it keeps defaulting to the old email. Help!

I don't have this issue with any other person. I have no idea why my phone keeps trying to attach this old email to this individual.