The best replacement for postmark is mailgun.

The best alternatives to postmark are rated as following:
★★★★★  mailgun
★★★★  evite
★★★★  mandrill
★  digital certificate
★  postmark
★  sendgrid
★  due date

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

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



Google Trends

Scale is based on the average worldwide traffic of postmark in all years. Click on graph for more.
postmark
mailgun
evite