OSX's mail program does have a bit of a reputation when it comes to mucking up its own SMTP settings.
If you have all the settings you need to hand - and only if - then this is something you can try:
1. Go to Mail > Preferences.
2. Click the accounts button, then within the accounts pane find and click the Account Information button. It doesn't matter at this stage which of your email accounts you're editing.
3. Click the drop down list for Outgoing Mail Server (SMTP).
4. From the bottom of the list, select 'edit server list'.
5. In the server list, select each one in turn and click Remove Server. ***Only do this if you are sure you have all the account info to hand***
6. When the list is empty, click done.
7. Go back to the Outgoing Server List (SMTP) drop-down, and this time select Add Server.
8. Enter the details for the SMTP server you want to use with your account.
9. Ensure this is the one that remains selected when you're done.
In essence, you're wiping everything and starting again because Mail has a habit of disliking it when you try to edit an existing SMTP server you already have set up. Don't ask me why, but we have been through this several times with my Dad's iMac and this always seems to fix it. Crude but effective.
Let me know how you get on.
---------- Post added at 21:14 ---------- Previous post was at 21:10 ----------
I have moved this thread as it's almost certainly down to the Mac's settings and not a Virgin Media email systems issue.