Outlook Express 5 for Macintosh downloads attachments to HTML mail without prompting the user, aka the "HTML Mail Attachment" vulnerability.
https://docs.microsoft.com/en-us/security-updates/securitybulletins/1999/ms99-060
http://support.microsoft.com/default.aspx?scid=kb%3B%5BLN%5D%3BQ249082