macOS Mail app displaying postal address next to email address
That is a Junk mail.

What they put in the TO field you can not change.

Somehow they have your Dads address in they records and put it in the TO field for some obscure reason.

However if you wish not to see emails like that then

Select that email

Click on Mail (top left) 

then Preferences

and select rules

Now set it up as shown, and emails from CheapOAir will be Automatically deleted. You can undo this if you want to see them again.