The best replacement for thunderbird is outlook.

The best alternatives to thunderbird are rated as following:
★★★★★  outlook
★★★★  evolution
★★★  apple mail
★★  pegasus mail
★★  outlook 2003
★★  microsoft outlook
★★  mail
★★  gmail
★★  mac mail
★  windows live mail
★  outlook express
★  postbox
★  eudora
★  firefox
★  windows mail
★  airmail
★  mail.app
★  zimbra
★  entourage
  opera mail
  em client
  outlook 2010
  seamonkey
  firebird
  outlook 2007
  mailbird
  classic 350
  live mail
  williams field
  windows live
  kmail
  desert storm
  street bob
  avenger
  electra
  inky
  outlook email
  mozilla
  bonneville
  avenger 220
  outlook 2013
  incredimail

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

Other search terms related to thunderbird and their ratings are:
★★★★★ exchange
★★★★★ gmail
★★★★ yahoo
★★★ hotmail
★★★ yahoo mail
★★★ lightning
★★ novell groupwise
★★ exchange server
★ google calendar
★ exchange calendar
★ owa
★ aol
★ webmail
google apps
myob
ms exchange
You have in mind a companion for thunderbird and it is not yet here? Propose!



Google Trends

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