Nothing definitive unfortunately!
Apple point to o2 or heart, heart point to apple or o2 and o2 point to apple or heart, in somewhat of corporate mexican standoff.
All have offered curtious, prompt customer service, with a between the lines subtext that reads: "we hope you get it fixed, and we want you to know we REALLY care about you as a customer, but it isn't our problem to fix - so go try those guys - it's probably their fault"
In fairness, a heart internet technician got me to attempt to send mails from 3g repeatedly over several port variations, whilst he looked at my account for activity to see where it was failing, and he said they couldnt even see it trying to connect at their end, and pointed to an issue with the phone or carrier, Given that the phones have changed across operating system version, handset model, and even manufacturer... im inclined to rule apple out at this point - but also I'm past the point of expecting customer support to offer answers after we establish turning the device off and on again doesn't fix anything.
Somewhat comically, all 3 companies followed up out of courtesy within a week or two, not to offer any further solution or suggestion, but simply to check if the other companies fixed it. I find this really makes all 3 feel less sincere, and more like their employees are just taught to use CRM software with callbacks/email reminders for customer retention.
All I know is that me and my business partner experienced the exact same problems, on different handsets, (iphone4/iphone4s/iphone5/ and now he has switched to an android phone - which is even worse for smtp send fails) we are both on o2 contracts but with different tarrifs. using the same heart internet SMTP mail server.
My most stable (yet convoluted) workaround was to install a webmail client on our server as a last ditch alternative. (it's called squirrelmail and you can find it in the heart internet backend) then use that through safari on the iphone.
Luckily, recently it seems to have been more stable, both on the UK data network and roaming abroad on my iphone 5 (iOS7) but for my business partners android phone, it's still as likely that email will send, as it is that Scotland will win the next world cup.
Whether this will work for you or not, i don't know, but I am using the below for SMTP.
Settings: mail.yourdomain.com
username: whatever@yourdomain.com
pass: (you'll have to fill in this bit)
Use SSL: Off
Authentication: Password
Port: 587
Sorry I can't be of more help!
