I was wondering why my home connection seemed to be crawling since yesterday. I didn’t worry too much at first, thinking that with my wife working from home yesterday as well, we were just downloading a lot of stuff between the both of us.

But the slowness persisted this morning. I took a peek at our bandwidth consumption and something became immediately obvious: there was something wrong on the upload end of things. We had somehow uploaded 2GB+ in one day, which is rather peculiar for us.

Turned out the issue at hand was an arcane bug involving Apple’s Mail.app “self-recovery”, MobileMe, Gmail, an oversized message and some kind of infinite loop.

1. Tried to send a 36MB video via email to a friend a couple of days ago.
2. Gmail prompted me that the message was too large, as they have a 34MB upload limit.
3. Mail.app kept trying to save it to my draft folder but likely kept getting the same Gmail error, silently.
4. Mail.app kept on “rescuing” copies of the said message in a “recovered messages” folder.
5. Mobile Me kept trying to sync my mail accounts.
6. Repeat ad nauseam.

I ended up solving the issue by simply trashing my entire ~/Library/Mail folder on my iMac, replacing it with a quasi-identical copy from my MacBook Pro and resyncing my mail accounts.

Problem now seems to be gone. Won’t get bitten by this one again…