Outgoing mail has to use the outgoing mailserver of the local ISP that you're connecting to. This is easy if you're using a dial up connection as you've presumably got the details off the ISPs website to enable you to connect in the first place.
It's a little more complicated when wireless roaming and I must admit I've never tried it because my ISP has a webmail site so I don't have to use Outlook when I'm travelling or at work.
I presume Virgin has a webmail site, if not you can use
www.Mail2Web.com or similar sites to send mail from your normal email account.