Just something that i have been pulling my hair out about but found the answer to it.
I use my own domain name with outlook and go through broadband with tiscali.
When sending any mail i was getting the email name is not on our list of known servers.
Here how to get round it. Delete the mail account in outlook and recreate it, When doing so it will ask you to type in the User Name and Password of you ISP account.
Password for your account.
If you have mail accounts allready set up this is something that when changing the allready setup account does not show. This runs in the background from the user and you can't see it. It only when you recreate the account in outlook that this shows up. And this is what you need to do in order to send mail after changing to Tiscali. (This is what i found)
Hope this helps some people looking for help as i did.