Well, I don't have any since I have no use for Tasker... but...
There are actually two plugins built-in:
1 - Lets you change the app's settings, account options (such as turning sync or push on and off, or adjust the mail sync interval), trigger mail check...
2 - There is an "event" plugin that can be set to trigger based on received messages.
They're visible in different places in Tasker's UI -- should be under "Plugins" both when you create an action (first plugin) and when you create a condition (second plugin).
