using another email service through TalkTalk

I have been using talk talk for some time with no problem, even with their email.
I also use another domain POP3 email and it has worked flawlessly and still does) for over 18months.
I wanted to change this domain to another and set it all up, and a new POP3 email address reflecting the new domain. I made sure the little box is ticked in Outlook 2007 saying "My Outgoing server requires authentication" and all of the settings are as required by my new domain.

For some reasoon the new email with that domain does not send to any webmail (hotmail, Yahoo etc) address but sends to POP3 addresses. My talktalk and original domain email continue to work perfectly.
Is this something that could be down to Talktalk and SMTP and if so, why is my original other domain address working perfectly - its just the new one?
Any ideas for starters will be real welcome. Thanks.

Are you sure that the outgoing server DOES require authentication? I though it didn't.