It is recommended you send file attachments of any type compressed to prevent errors while transferring. This is from a Mac or PC. If you're sending to PC users, there are plenty of programs for Mac OS X and earlier that will zip the files so PC users can easily expand them. As for the problem You've described, I've only seen this occur when sending from a PC using Outlook Express with the "break messages apart larger than x kb" checked. I can't find a similar option in Mail.app though.