Man I am having the same freaking problem, did you by any chance try this method before the BIS 2.5 update?
3ws blackberrysync KOM/2008/03/how-to-get-hotmail-on-your-blackberry/
remove spaces above
I noticed when I sent an email to my work address typed in and not from my address
book (but i get that email on my phone), it said it was sent from:
myphonenumber@messaging sprintpcs com
weird. I then tested sending to a yahoo account (which I also get on my phone) and it
said it was sent from my hotmail address like it should. WEIRD maybe it has
something to do with pop/imap settings? I tried the method in the link above before
BIS 2.5 i will go back in undo what I did and see if that works. Guess I will just have
to delete some email address from my address book.