YES email has been an issue since early days--- the Late Night Data usage reporting isn't an issue and has not been since AT&T changed how the reporting behaves-- as was pointed out prior in this thread there were multiple reports jamming up what appeared to be a billing error--- that behavior is gone and agree that this thread is so long and full of "behaviors" that many viewers do not take the time to read the thread.
Back to the email behavior-- yes, and it depends upon which iPhone model you have. This is something else that posters are ignoring. Original iPhone users like me are stuck without the tools that are being added in each new iOS upgrade. My email takes forever and even after the mail is downloaded as posted previously, the Mail app continues to consume data as do the other apps. This was confirmed by at least one app developer.
There is still a huge hole in the AT&T billing practices that an intimate poster made known. AT&T will not devulge their billing practices and I don't see the FCC pressing them to do so.
As for "security" there is ongoing concern that SuperCookies and app providers data collection consumes data--- who pays for it? The same early concern was voiced with Apple and the opt out process is confusing to some degree depending upon which iPhone you have, whether you use a PC or MAC and finding it to disable it. The other security concerns are twofold, a)AT&T security and since I have a record of spoofing of my iPhone and an AT&T case--they did credit all the calls back to me and usage and even overcompensated my account with minutes and b)WiFi access and potential security vulnerabilities.
Then, the behaviors of when, why and how the iPhone consumes data. It is absurd for Apple, AT&T and app makers to assume that users are okay with the presumptions that the iPhone is an "always on, always consuming data device" per the AT&T response to the FCC. This is their bailout phrase or disclaimer. Everyone wants to sell an app, service or the iPhone and yet users all want and demand mobility. The opportunity is the biggest fleecing of consumers-- bigger than ripping them for texting which is pure profit.
So in short, it's not just an IMAP issue and an emphatic, NO - it's not a "user issue" either. It is seemingly complex and contradictory as to reported symptoms/fixes/finds/observations; while IMAP is just one more thing contributing to the root cause of customer data limits being whacked out when users know they don't use the apps heavily and have gone to extremes to disable every possible thing under their power. My iPhone exhibits the same behavior with Safari as in Mail. After a webpage is downloaded, it continues to spin and spin--- seemingly a completely loaded webpage but data errors, retransmits or whatever going on in the background that equates to higher than expected data consumption. Couple these behaviors with what the app developer said about how long any app continues to run and then mix in AT&T or any "Carriers" billing practices that they DO NOT REVEAL and you get insanity.