The best replacement for postfix is exim.

The best alternatives to postfix are rated as following:
★★★★★  exim
★★★★★  sendmail
★★★  dovecot
★★★  qmail
★★★  suffix
★★★  exchange
★★  prefix
★★  exim4
★★  courier
★  sendmail ubuntu
★  sendmail centos
★  unary
★  prefix increment
★  ssmtp
★  prefix java
★  postcrete
  opensmtpd
  powermta
  exim ubuntu
  citadel
  infix
  zimbra
  ms exchange
  courier mta
  exchange server
  apache james

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

Other search terms related to postfix and their ratings are:
★★★★★ dovecot
★★★★ sasl
★★★★ spamassassin
★★ tls
★★ multiple domains
★★ sendmail
★ procmail
★ ses
★ smtp auth
auth
clamav
squirrelmail
smarthost
You have in mind a companion for postfix and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of postfix in all years. Click on graph for more.
postfix
exim
sendmail