Yesterday, for the first time on 3 months that I'm a Gmail user, I've tryed to auto-sent me a .ZIP attach with a source code of a Visual Studio 2003 solutions I've developed on my office.
What was the result? Gmail has refused it... I was asking why this, and after few try, I've discovered a noise thing: if your .ZIP file contains some executable code, such as .EXE or .DLL, the Gmail system recognize it as an invalid attachment and refused it. If your .ZIP don't contains these types of files, it's accepted.
Now that I know this, I send me .ZIP files with executable code on them simply by renaming the extension (for example, .ZIP will become .ARC, archive 
). This works good but, as you can see, it's really noise...
Google, why this choice? Gmail is a beta service, maybe you can review these settings...