First, I've got around this problem, so I'm not desperate for an answer, but I thought I'd share it as it's a bit weird. I'd be interested in anyone's thoughts though.
I use Windows 7, with Windows Live Mail with a Gmail account (using IMAP). I've got Panda free antivirus installed.
Yesterday, I was asked if there's a way to tell if a PC has been switched on while you weren't there. There is - a nifty little utility from Nirsoft. (I recommend a peruse around the site, there's loads of handy things there - eg I use wakemeonlan quite a bit, not so much for waking remote computers, but for scanning the lan and easily finding the ip address allocated to a particular device) http://www.nirsoft.net/
The program in question here has a mildly unfortunate name - TurnedOnTimesView.
I wrote a quick email, and attached the zip file I'd downloaded from the Nirsoft site, and sent it. No errors appeared.
Later, I was told that the email hadn't arrived.
I checked in my outbox and sent items - the email wasn't there! I was sure I'd sent it. So I sent it again. And looked in my sent items. And it wasn't there! I sent another plain email, that correctly appeared in sent items once it had sent.
The file passes a virus scan with no problems. But, in any case, the antivirus didn't tell me it had blocked it. And there's no sign of it doing so when I view the Panda antivirus log.
The zip file is 67K, so I don't think I'm exceeding a size. (In any case, read on - I did a few tests..)
I unzipped the file, and recompressed it using 7zip, and set a simple password for it (also on the filenames). The 7zip file was called TOT.7z
I emailed this, and it appeaers to have worked - it showed up in my sent items.
Wanting to prove that it was the naming that was causing problems (perhaps a spam filter somewhere?) I then created a plain text file called "test document.txt" with "this is a test" in it. I then made four zip/7z files of this, with differing names. None of these zip/7z files were password protected.
Here's the baffling results :
test document.zip sent ok
turnedontimesview.zip sent ok
test document.7z sent ok
turnedontimesview.7z disappeared [edit : has just appeared in sent items 30 minutes late!]
OK, so now that last test has appeared it means this test hasn't proved much. More tests to be done I think. The two attempts at sending the original zip file still haven't appeared in the "sent items" list.