This is a list of mail server software: mail transfer agents, mail delivery agents, and other computer software which provide e-mail within Internet message handling services (MHS).
List of mail server software Wikipedia
According to one survey, sendmail, Microsoft Exchange Server, Postfix, and Exim together control over 90% of market share for SMTP service in 2014.
Another survey suggests a more balanced distribution, though it included hosted e-mail services such as Postini.
These surveys are rather difficult to produce, hence it is no surprise if they disagree.