Why are emails now sending as @me

Hi,

Since i installed lion on my macpro and macbook pro, when i send an email now they are being sent as xxxx@me.com - whereas before they were sending as my normal email address xxxx@mac.com

Why is it doing this now? I just want to be able to send and receive mails using my @mac.com email address, and i want my mail to be consistent on all my apple devices - mac pro/mbp/iphone4S

It's really starting to annoy me now how much hasstle this is turning out to be.

thank you for any help and advice.