The best replacement for mac mail is outlook.

The best alternatives to mac mail are rated as following:
★★★★★  outlook
★★★★  entourage
★★★  gmail
★★★  thunderbird
★★  outlook 2011
★★  airmail
★  sparrow
★  outlook 2013
★  outlook 2010
★  entourage 2008
★  mac outlook
  microsoft outlook
  microsoft entourage
  apple mail
  mozilla thunderbird
  outlook for mac

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

Other search terms related to mac mail and their ratings are:

★★★★★ hotmail
★★★★★ gmail
★★★★ exchange
★★★★ yahoo
★★★ exchange 2003
★★ google apps
★★ exchange 2010
★★ office 365
★★ exchange server
★★ aol
★★ yahoo mail
outlook web access
multiple accounts
microsoft exchange
hotmail imap
godaddy email
gmail imap
yahoo settings
yahoo plus
You have in mind a companion for mac mail and it is not yet here? Propose!

Google Trends

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