You will need to go to the "account" settings of your email software (e.g. Outlook Express, Windows Mail etc.) and change the SMTP server details.
In Outlook Express to do this go to Tools, Accounts, then select your email account, click Properties, click on the Server tab and change the Outgoing Mail (SMTP) value.
In fact, it's probably best to set up a second set of account details (just in the your email software) which is exactly the same as the first except for the different SMTP setting. That way, when you are out you can select to send from the T-mobile one and at home you can use the Virgin one.