Weird problem: Default email changing

hey everyone i've got a weird problem.

I have my default services (calendar and messaging) set to my tmobile email. yet, when I try to email some people, it says "send using default email". When i start typing the email, it changes the "default" to my work email address.

When i tried emailing my wife, it changed to my work email.

but when i emailed my brother, it was the correct tmobile email.

help please, its annoying!

If you have sent or received emails to/from people in the past using one email address it will select that email as the default email.