I’ve mostly been using msmtp as a mail transport agent on my linux systems but I wanted to review what was available:

• nullmailer – 2.1 released 2017-Oct, nullmailer on github
• msmtp: 1.6.6 released 2016-Nov, msmtp on sourceforge
• dma – 0.11 released 2016-Feb, dma on github
• ssmtp – unmaintained
• esmtp – unmaintained

My preference is something hosted on github given the ease through which issues can be reported and tracked.