GrowlMail, the Mail-specific notification bundle for Growl doesn’t work under Leopard, although the next version (1.1.3) will work. If you are missing it and can’t wait, this applescript workaround will help.
Kevin Way posted an AppleScript on his web site two months ago that can be attached to a Mail.app rule.
It passes notifications of new messages directly to Growl, and since it is rule-activated, you can use conditions to make it tell you only about emails from particular people. Of course, you could also set it to match on particular words in the subject of the mail, or only for emails from a particular account, or for whatever other condition you set in the rule.
I found that the latest beta of MailTags stopped me creating a rule with an applescript attached, but uninstalling it, creating the rule and then loading MailTags again seems to work fine.
The end result is a nice Growl alert:
A thread on the Growl forum contains more tips and tricks for tweaking the script.
Tags: Apple Mail, Apple Mail Tips, applescript, growl, mail.app, notification, rules