The easiest way to deal with the problem is to completely delete your account details and start again. (That doesn't delete any existing mail. It's just the settings - which appear to have become corrupted - which are deleted and then recreated).
Go to Tools > Accounts > Mail.
Click on your email account, to highlight it.
Click on 'Remove' and then on 'Yes'.
Then click on Add > Mail and create the account again. You'll need to know your username, your password and the POP3/SMTP server addresses. For Virgin, the incoming (POP3) address is pop.virgin.net. The otgoing (SMTP) address is smtp.virgin.net
Don't worry if you've never manually created an email account before. It's an absolute doddle. You just follow the on-screen instructions.
Chris