"Exim is a mail transfer agent (MTA) used on Unix-like operating systems. Exim is free software distributed under the terms of the GNU General Public License, and it aims to be a general and flexible mailer with extensive facilities for checking incoming e-mail.
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A large number of Exim installations exist, especially within Internet service providers[1] and universities in the UK. Exim is also widely used with the GNU Mailman mailing list manager, and cPanel."
Take an example from earlier today: there's a vulnerability in Exim that can allow arbitrary remote code execution (http://www.vupen.com/english/advisories/2011/1185). For anyone who doesn't know what Exim is, I refer you to Wikipedia (http://en.wikipedia.org/w/index.php?title=Exim&oldid=421...):
"Exim is a mail transfer agent (MTA) used on Unix-like operating systems. Exim is free software distributed under the terms of the GNU General Public License, and it aims to be a general and flexible mailer with extensive facilities for checking incoming e-mail.
...
A large number of Exim installations exist, especially within Internet service providers[1] and universities in the UK. Exim is also widely used with the GNU Mailman mailing list manager, and cPanel."