I am not losing email, I am not losing email, I am not losing email ...
I checked my web mail account (from which I am normally downloading all mails with Apple Mail and hence almost never get to see the site) and there was the spam message that I saw being downloaded and then disappearing. It was still sitting in my inbox there, marked as unread. So I guess it was just repeatedly downloaded every few minutes, causing that error message to appear all over again. After deleting the email through the web interface, the error disappeared (relief!). So what was wrong with that email? It did not display in the web interface either (although it had a few KB listed for its size), so it was probably seriously broken (perhaps intentionally to trick spam filters), which means that I am not losing mail.
At least I am not losing mail because of my mail application. I sometimes think that emails which I write disappear on the way to their recipient, especially when posting to mailing lists. I do not want to think too much about that.



