The best replacement for airmail is sparrow.

The best alternatives to airmail are rated as following:
★★★★★  sparrow
★★★  postbox
★★★  surface mail
★★★  apple mail
★★★  ems
★★  inky
★★  unibox
★★  mac mail
★★  thunderbird
★★  sal
★★  mail
★★  courier
★★  mail pilot
★  mail mac
★  airsure
★  airmail
★  mailplane
  mailmate
  air parcel
  international signed for
  mail.app

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

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



Google Trends

Scale is based on the average worldwide traffic of airmail in all years. Click on graph for more.
airmail
sparrow
postbox