if you want to use your TalkTalk e-mail account with an Apple iPhone then set up your phone using the the standard TalkTalk e-mail settings for incoming and using O2's server for outgoing.
When connected to EDGE, the mobile data connection, you can send and receive e-mails using this configuration.
The only issue you will have is when you try and do the same when connected via WiFi as you will not be allowed to access O2's outgoing mail server to send e-mails. Receiving e-mails will still work. To get around this you will have to temporarily switch WiFi off.
This appears to be an oversight by Apple and O2 and hopefully they will fix this issue in the future.