Being the market leader in Enterprise mail services isn't all that big a deal. Enterprise mail services are a niche market.
Let me be explicit - The majority of email servers are run by ISPs and Web Hosting services. Relatively few Enterprise class mail servers exist outside of major corporations (most small and even medium size businesses get their mail from their ISP or Web Hosting service); Enterprise mail is a niche market.
Most ISP and Web Hosting services run Linux and therefore probaly use postfix, dovecot or some similar mail server (and probably sendmail). Most also use ClamAV as their inline AV engine for mail. Depending on how you interpret the percentages, Linux has between 2/3 and 3/4 of the server market in general, and the percentages are even higher on the Internet core backbone. That's a pretty large professional marekt for ClamAV.
@MadMacs0 - I chose not to mention OS X server because it's such a tiny percentage of the server market share. Really, the point is that calling ClamAV a poor choice for professionals (which is code for "when it really matters") is demonstrably false when the professionals who run the Internet itself use ClamAV.