The best replacement for apple mail is outlook.

The best alternatives to apple mail are rated as following:
★★★★★  outlook
★★★★★  thunderbird
★★★★  gmail
★★★★  entourage
★★  airmail
★★  outlook 2011
★★  postbox
★  sparrow
★  gmail app
★  entourage 2008
★  google mail
★  outlook 2011 mac
  mac outlook
  mozilla thunderbird
  eudora
  microsoft entourage
  microsoft outlook
  outlook 2013
  thunderbird 3
  outlook for mac
  mac mail

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

Other search terms related to apple mail and their ratings are:
★★★★★ gmail
★★★ exchange
★★★ hotmail
★★ yahoo
★★ google apps
★★ exchange server
★★ exchange 2003
★ gmail imap
★ yahoo mail
★ aol
★ godaddy
growl
You have in mind a companion for apple mail and it is not yet here? Propose!



Google Trends

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