Emailing from a Web Page...

A friend of mine used to have xyz internet and changed to abc internet. He emails people a lot from their websites. When the email pops up to do so, it still says "From: SoAndSo@xyzinternet.com". He wants to know how he can change that to read abc internet. I know it's a silly question but he and I have looked everyplace and removed all (we think!) references to xyz internet.