I’m not a Mail.app user, simply because there’s no reason for me to have my e-mail locally – I much prefer to keep it “in the cloud” on Google’s servers. But it is up to you how you want to organize your e-mail. Obviously, if your provider only supports POP3 and no web interface, then you’re pretty much forced to use a client. Probably the best mail client for Mac is the one that ships with Mac OS X: Mail.app.
Mail is a great app, but lacks some useful features. Luckily, MailTags is an app that will bring an extra useful feature to Mail.app. It is now in Beta 4 (still a little buggy). I would consider this the ultimate Mail.app enhancement. Tags are basically a part of everything on the internet. It’s a powerful way of organizing anything – and it works well. MailTags does pretty much what the name says – it allows you to put tags on e-mails, as well as much more. It also compliments other shareware applications like iGTD, MailSteward, TagBot, EagleFiler, DEVONthink and Pagico.
Via [TUAW]
It´s better use gmail´s imap service, tags in gmail are folders in Mail, all connected with no needs os extensions.