If you want to add tags to your email in apple mail, and search tags using spotlight, this free MailTags plug in is superb.
With MailTags you can add a variety of metadata to your messages:
- Add multiple keywords to your messages. Build your own "folksonomy" for your email as you read your mail. Some useful tags might include "To Do", "Done".
- Set a project or category for message. This is more than just filing a message in a folder because it allows you to cross-categorize messages. You may have a mail box for action items, but you want a way of specifying the project that the message relates to.
- Set a due date on a message. Too often we have messages that need to be responded to by a certain date, or they contain actions that, while cannot be acted on immediately, need tickling on certain dates, However, in most email programs it is unwieldy to create and manage "tickle" folders to make sure due dates don't get missed.
- Add notes to messages. With notes you can add your own personal comments to messages that could record what was in your head at the time of triaging the message or what things need to be done to act on this message.
- Set your priorities. While Mail does display a messages priority. That priority is set by the sender, and amazingly enough, it can't be changed! MailTag adds a priority tag so you can decide what is important.