googlemail won't send

I have just bought an iphone and found problems in sending from my old email address with ntl. I have set up an account on googlemail and this is great for my iphone. My problem is that I can't send email from my IMac although I can receive them with no problems. I have checked the addresses and everything seems ok but I keep getting "The sender address [email protected] was rejected by the server smtp.googlemail.com. " I want to keep both email addresses for a while.
Any advice ???.

Hi all
I finally found what I'd done wrong. I hadn't fully completed my information on the Google website. This wasn't a problem for my iphone but my Mac didn't like it. It is now sorted.