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.
[1] As of December 2023[update], 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][a] 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.