This is a list of mail servers: mail transfer agents, mail delivery agents, and other computer software which provide e-mail services.
According to one survey, sendmail, Microsoft Exchange Server, Postfix, and Exim together control over 85% of market share for SMTP service.
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.
This list is incomplete; you can help by expanding it