problem sending o2 bt connect emai using iphone 5 with wi-fi but working on 3g

[supertown81]

strange one - I have an iphone 5 on O2

I can send and receive emails on my bt connect ( on O2 ) Ac on the phone using 3g

however when I use it on wi-fi it it will receive emails but not send.

But on my Bt.internet.com ac also set up on the phone it will send and receive using 3g and Wi - Fi

Help ???