Note: All the circumstances involved IMAP email acocunts. The details below explain why.

What was happening back then was that the iPhone would get stuck on a particular email and repeatedly try downloading it. For those unaware the sequence of operations the iPhone follows to get new email is as follows –

1. Get header list of new emails, i.e. subject, from, date
2. After listing the header information for each new email,
3. loop through those new emails and download the two-line summary of the body of each email

It was this third step that caused the problem. The iPhone was failing to get the summary of the body of the email and would then re-request the same from the mail server, and then again, and then again, and then again…

This could and did happen THOUSANDS of times when the problem occurred. As far as the mail server was concerned these were all valid IMAP requests. As the iPhone was CONTINUOUSLY using its WiFi or 2G (this predated the iPhone3) transmitter this caused the rapid battery drain and the iPhone to get hot.

I was able to confirm this behaviour by monitoring the mail server logs and seeing the same IMAP client (the iPhone) making the same IMAP body summary request for the same message ID thousands of times.

At least one of the mail server vendors eventually said there had been a bug in their software but having also reported it to Apple I got the impression that Apple also had some aspects to fix in the Mail application on the iPhone.

I also got the impression Google GMail had the same problem for a short-time but appeared to fix it much quicker so that I only reproduced it briefly with a GMail acocunt.

While I have not seen full technical details of the Windows Mobile 7 and Yahoo problem that superficially sounded very similar, it could have been a similar issue.

The reason I felt this occurred specifically on the iPhone back then was that the IMAP body summary command seems to be rarely used in other systems and is not as far as I am aware used in Apple Mail in Mac OS X as an example. Prior to the iPhone mobile phones were probably to dumb to comtemplate using such a command. With (theoretically) this IMAP command being so rarely used it may well have exposed previously unknown mail server bugs. I also suspect it was related to the structure of the trigger emails – perhaps these emails subtly broke various email rules. With so many different email packages in existence a surprising number of emails do slightly differ from the official rules.

As an example, Apple’s own Mail application has in the past broken the rules by putting attached files inside the HTML format section of the email, rather than correctly listing them in their own section. This resulted in some email clients which defaulted to viewing the plain text version then not being able to see the attached files at all.

Yes, it was an Exchange account. If it happens again, I may try some experimentation to see what email caused it.

I still don’t know exactly what the issue was, but I suspect it was something with an email I received – and no, I didn’t have the time to go through one-by-one and test, unfortunately – but at least now there’s one more avenue to check for anyone else who experiences this problem.