The best replacement for lotus notes is outlook.

The best alternatives to lotus notes are rated as following:
★★★★★  outlook
★★★★  exchange
★★★  sharepoint
★★★  microsoft exchange
★★  microsoft outlook
★  domino
★  outlook market share
★  outlook 2013
★  outlook 2010
  exchange server
  exchange 2010
  outlook better
  microsoft office
  outlook express
  lotus domino
  ms exchange

Your alternative for lotus notes is not listed? Suggest!
Speaking about lotus notes, one probably shouldn't forget about google calendar.

Other search terms related to lotus notes and their ratings are:

★★★★★ google calendar
★★★★★ outlook 2010
★★ palm
★★ sametime
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lotus notes