When someone send us an email with attachemnt from a gmail account we do not receive it at all. When that someone sends and email from a gmail account without attachement we receive it.
The Antispam on exchange is disabled. Also the antispam on the server antivirus is disabled. That's why we beleive has something to do with our Firebox.
Follow that
http://www.watchguard.com/infocenter/editorial/18360.asp