The best replacement for mail.app is thunderbird.

The best alternatives to mail.app are rated as following:
★★★★★  thunderbird
★★★★  gmail
★★  entourage
★★  sparrow
★  airmail
★  postbox
★  outlook 2011

Your alternative for mail.app is not listed? Suggest!
Speaking about mail.app, one probably shouldn't forget about exchange.

Other search terms related to mail.app and their ratings are:
★★★★★ exchange
You have in mind a companion for mail.app and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of mail.app in all years. Click on graph for more.
mail.app
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