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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.

Exim configuration

Exim is highly configurable and has features lacking from other MTA’s. It has always had substantial facilities for mail policy controls,providing facilities for the administrator to control who may send or relay mail through the system. It is matured to an access control list based system allowing very detailed and flexible controls and allows easy integration of anti-virus and anti-spam measures which made exim very suitable for diverse mail policies.

Modes of operation

Access Control List (ACL): which defines behaviour during the smtp sections that includes transfer, receive and routing.
Router section: which includes a number of processing elements on address of mails during the time of delivery.
Transport section: which includes processing elements that transmits real messages from source point to destination point.
Retry section: where policy on retrying messages that fail to get delivered at the first attempt is defined.
Rewrite section: defines if and how the mail system will rewrite addresses on incoming e-mails.
Authenticators section: with settings for smtp auth , a rule per auth mechanism which means smtp client login using an authentication mechanism that supports smtp servers.

Configuration file : /etc/exim.conf
The exim.conf conains all exim modes of operation.

Log files

/var/log/exim_mainlog:  contains all interactions that exim handles, so both incoming and outgoing mail transactions.
/var/log/exim_rejectlog: contains all connection attempts that were denied.Also it can be checked in exim_mainlog file.
/var/log/exim_paniclog: contains unexpected/fatal error log ie; IMAP, POP authentication failure (login attempts),transactions and spam scoring.

Restarting exim
service exim restart  or /etc/init.d/exim restart

Status of Exim
service exim status  or  /etc/init.d/exim status