Here's the info I've previously read. It indicates zip files are ok, but I'm
pretty sure I've had trouble with them in the past x-ferring access files.
http://office.microsoft.com/en-us/assistance/HA011894211033.aspx
Also, this quoted directly from the help files:
To provide enhanced security, Microsoft Office Outlook 2003 is designed to
prevent you from unblocking attachments.
Because Outlook is so widely used, it has been the target of several virus
attacks in the past that have affected millions of people. Microsoft has
acted to protect people from files, such as .exe and .bat files, that are
often used to run malicious scripts when opened. Unfortunately this makes
file sharing less convenient for many people, but security must take
precedence.
Microsoft Office 2003 file types most commonly shared between people, such
as .doc, .xls, and .ppt files, are not blocked.
If you need to share files that have file types blocked by this feature, you
have several options, including the following:
Rename the files to include a temporary file type that is not on the list of
blocked file types. For example, you might rename MyFile.exe to
MyFile.exe_EXTRA, and then attach the file to the e-mail message. You can
include instructions in the message to save the file with the correct name,
for example, MyFile.exe, when the recipients save the file to their computers.
Use a program, such as WinZip, to package files before you attach them to
your e-mail message. In your message, you can include instructions explaining
how to extract the files from the package to make it easy for recipients to
access the files.
Post the files to a secure network share. In your message, you can include a
link to the share that you have given the recipients access to.
If your organization uses Microsoft Exchange Server, your e-mail
administrator can control which file types are blocked. For more information,
contact your administrator. If you are the e-mail administrator, you can
learn more about configuring blocked attachment behavior in Outlook by
referencing the Microsoft Office 2003 Editions Resource Kit in the online
Deployment Center. See Configuring Outlook Security Features to Help Prevent
Viruses.
CW