Sent mail occasionally vanishes (but is sent)

Question

I’m getting an intermittant (ie: hard to pin down) problem on both of my IMAP accounts, Sometimes, when I send an email, the message is sent (I get the whoosh noise, and the recipient does receive it) but my copy disappears without a trace. Not stored locally, and not stored on the server (I’ve checked via webmail). As somebody who keeps every email either sent or received that is part of a client conversation, this is a problem, as I now have gaps in my permanent record.
Just to cover the basic questions:
Yes, I have the account preferences set to store messages on the server, and never to delete them;
I am unable to determine any particular trigger for this problem;
I have a broadband connection functioning at a reasonable speed;
Mail 4.5, OS X 10.6.8;
The problem occurred under Leopard, too;
Other threads on these discussion forums reference the same problem; often the  discussions start to focus on the problem being linked to whatever specific ISP the original poster is using, but with almost everybody who posts this problem having a different ISP/mail host, I am of the opinion that this is simply a Mail bug, independant of specific ISPs or mail servers. Also, none of the threads have come up with a satisfactory solution AFAIK.
If anyone has anything they can suggest to correct this issue then please chime in, because it's becoming an increasingly problematic phenomenon, and seems to be getting more frequent.
Thanks;
Rick

Answer 1

Months further down the line and this glitch/bug is still happening with no
discernible pattern or trigger and I'm no nearer finding out how to prevent it -- or even if that is possible. For the record, I get the same error message appearing in Console as Marco does:
Error (null) occurred while trying to append messages to outgoing store. Ignoring and proceeding with delivery...
...and goggling the problem shows that we are not alone... but nobody has come up with a remedy. From what I've been reading, this problem seems to have cropped up in Leopard and continued into Snow Leopard (I can't vouch for Lion; I'm not using that yet).
For anybody who uses email as a comprehensive permanent record of business communication this bug is critical. And if it is a bug in Mail (I say ‘if’ because it could be a conflict with a particular flavour of mail server or something) then it does rather send out the message that “this is fine for casual consumer use, but if you need your email to work in a reliable, professional context then look elsewhere, because Mail ain’t really for you”.
Which is a shame, because other than that (and a few other feature-set deficiencies) it's a good email app.