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 December2023[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.
↑ The following server types are mentioned in the Mail (MX) Server Survey but are not represented here - ArGoSoft, GroupWise, InterScan VirusWall, MagicMail, MessageWall, ModusMail, Neon Mail Server, OpenVMS, Post.Office, Trend Micro, VisNetic.
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