Open-source MTA

An open-source MTA is a mail transfer agent whose source is freely available under an open-source license: historically Sendmail, Postfix, Exim, and qmail for general mail service, and more recently projects purpose-built for specific niches (high-volume outbound, SMTP frameworks, all-in-one servers). Open-source MTAs eliminate license fees and allow unlimited inspection and modification; the operating expertise still has to come from somewhere.