Exim Exim is a mail transfer agent which is used on Unix-like operating systems for sending,receiving and routing the email messages. Exim is a free software distributed under the terms of the General Public License (GNU), and it aims to be a general and flexible mailer with extensive facilities for checking incoming mails. The mail transfer agent exim is developed in 1995 by Philip Hazel at the University of Cambridge.

Your MX (Mail Exchange) records define where your email will be routed to. When an email is sent, the MX records are looked up publicly, and mail is routed directly to that server.