It is usually recommended you use the one they supply for the first 14 days. Some form of line optimisation takes place I believe. Then swap back to your own if you want.
Remember whatever you do that you have to return theirs at the end of your contract. Else they bill you �100 for it.
Last point, you don't get free email with Be. They charge something like �2 per month for it, and apparently it is pretty ropey.
Better to get ISP-independent email straight away. Then you can tell all your contacts well before you migrate, and watch out for any you have forgotten. Then you never have to change email address again.
Googlemail is free and a lot of people like it. For something a bit better there are several email providers. Click Purple Cloud in my sig to see mine. Your own .co.uk or .me.uk, (which has more availability), costs �7.50 every 2 years, and email to go with it is �5 per year. Five mailboxes, unlimited aliases, POP3/SMTP/IMAP/Webmail. Spam Assassin on the servers.
Or instead of the email web space starts at �10 per year and includes the email above.