"Performance tests conducted by Sendmail and IBM found
that a single IBM eServer z900 mainframe could support more than 2
million users, making it the largest single-server e-mail system on
Linux currently available, according to Sendmail.
For enterprises that require larger environments, Sendmail
offers a cluster architecture that enables customers to harness
multiple mainframes to transparently serve a greater number of
Sendmail believes the products' seamless scalability and high
reliability make the solution for the IBM eServer z900 ideal for
global enterprises with mission-critical e-mail systems supporting
employees, customers, and partners, as well as for service
providers that host e-mail services."