Hi pjp,
I'm afraid that it's common practise these days for your ISP to prevent you from connecting to their email servers if you're not currently using one of their internet connections. This is why it will work from your TalkTalk line, but not from Vodafone (or any other provider).
Usually you should still be able to receive emails, so it seems that their blocking is particularly stronger than usual.
Unfortunately there is no real solution to your problem, other than using a third-party webmail account, such as
GMail, and having your TalkTalk emails forwarded to your GMail address instead.
I trust this helps
Jon
eForum Team