If you are running Mojave you need to install our new MailSuite app to get MailTags 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 MailTags 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