WinZip E-Mail Companion is a handy application that I just gave a try. Do you know of some free alternatives that maybe supports 7-zip?
WinZip® E-Mail Companion Beta makes it easy to zip (compress) the documents and other files that you attach to Microsoft® Outlook, Outlook Express, and Windows Mail e-mail messages. Zipping attachments saves transmission time and disk storage for both the sender and receiver of the e-mail.
If you want to try the beta download it from here: http://www.winzip.com/betaout.cgi
Update. I found that on Vista and Outlook 2007 the Configure settings are not retained. After contacting WinZip they explained that it is only when accessing configure from within an email message. When using Tools/Options/WinZip E-Mail Companion it works. Hopefully they will fix this in due time.