This is a list of mail server software: mail transfer agents, mail delivery agents, and other computer software which provide e-mail.
All such figures are necessarily estimates because data about mail server share is difficult to obtain; there are few reliable primary sources—and no agreed methodologies for its collection.
Surveys probing Internet-exposed systems typically attempt to identify systems via their banner, or other identifying features., Postfix and exim appeared to be the overwhelming leaders in mail server types, with greater than 92% share between them, having come to prominence before 2010 in each case.[1] While such methods are effective at identifying mail server share for receiving systems, most large-scale sending environments are not listening for traffic on the public internet and will not be counted using such methodologies.