EMails won't Send in iMac and iphone

I have running an IMac, iPad2 and iPhone 4. I receive my emails to all 3 devices but I can only send emails via my iPad2. This fault has only just occurred (although granted I had never tried sending an email via the iphone previously). When i try to send an email via the iMac or iPhone I get this error message

"The sender address [email protected] was rejected by the server smtp.talktalk.net." where xxxxxx was my username.

I have looked under settings and all seems ok.
The only other clue I have is that this occurred after I noticed a "server outage" notice on the talk talk website which appears now to be fixed.
I have contacted Apple Chat support and talk talk chat support without any resolution to this problem.
Any ideas anyone?
Bill

Is your outgoing port set to 587?

My Outgoing port is set to (25, 465, 587)

Hmm seems if I set the outgoing port to 25 on its own then all 3 devices work. Here's Hoping!!