The best replacement for smtp is pop3.

The best alternatives to smtp are rated as following:
★★★★★  pop3
★★★★  mapi
★★★  pop
★★★  imap
★★★  sendmail
★★  exchange
★★  esmtp
★★  php mail
★  smtp
★  smtps
★  http
★  pop3 server
★  smpp
★  ftp
★  pop server
  mime
  exchange server
  sip
  api mandrill
  smtpout
  x.400
  snmp
  x400
  pop mail
  snpp

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

Other search terms related to smtp and their ratings are:
★★★★★ tls
★★ authentication gmail
★ phpmailer
You have in mind a companion for smtp and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of smtp in all years. Click on graph for more.
smtp
pop3
mapi