If you are running Mojave you need to install our new MailSuite app to get Mail Act-On functionality. With Mojave, we have consolidated our 4 plugins into a single product with a single installer. You do not have to install all components of MailSuite if you only want to use Mail Act-On but you do have to do the installation through the MailSuite app.
Mail Act-On is the powerful enhancement to Apple's Mail Application that puts organization at your fingertips. Use Mail Act-On to create rules to perform actions on messages by responding to simple keystrokes. Create Outbox rules to automatically add headers and set the archive mailboxes of sent mail. Mail Act-On is the indispensable tool for anyone who needs to manage their mail more efficiently.
Note: Although Finder indicates that this is version 1.0, the developer states that this is version 4.1.6