MailMate 1.5.1 adds support for tags:
The implementation of tags is based on IMAP keywords which means that they are easily and efficiently synchronized with an IMAP server (some servers may have limited support). The user interface is an unobtrusive token based text field shown when hitting a simple shortcut. It is very similar to the recipient address fields of the composer including automatic completion of tag names. It is easy to create new tags and a new preferences pane allows you to review and edit existing tags and their IMAP keyword equivalents. Read more about tags in the manual.
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