We've got a little weird problem. When either forwards or send an email with attachments from native mail app on an iOS device, emails are not sent and remains in the outbox. This happens only when you are on 3G / 4G, works fine when connected to WiFi. My company uses Exchange with Office 365 subsciptions. Have tested with Android and it works fine. Suspecting that there is a kind of bug with native mail app and Exchange, but is not sure.
It's quite possible it is in ActiveSync bug on the native iOS mail client - there have been those before. What if you try Outlook on iOS? That would determine if it's the iOS or the fact that it's on a mobile network.
I had an issue very similar to this a few years ago with our hosted exchange platform here at Rackspace.
Ultimately it was an escalation that went through AT&T and was related to some local blocking rules they had with message sizes. Unfortunately this was not something a level one tech could help with, and it took us a few weeks, and escalation throught he Coorporate AT&T rep to get to the bottom of it.
I dont think that helps you solve the case directly here, but just wanted to share my experience.
I think for next steps:
1. Download the native outlook email app, or perhaps a third party one like touchdown. Setup the mail account there, and see if you have similar problems.
If you do not, then it is not a network issue, and you can focus your troubleshooting on the APP.
If you do, then it is probably a phone/network issue, and something you would need to involve your telecom vendor in.
Either way this step helps clear the path for how you move forward.