Get exim to listen on another port on top of port 25
Some ISP's are now blocking outgoing port 25 which prevents user from using smtp via their server. The workaround is to get exim to listen on another port other than 25 to bypass the ISP's block.
For example, to get exim to listen on both port 25 and port 587, you'd add the following code to /etc/exim.variables.conf.custom file:
daemon_smtp_ports = 25 : 587
Once saved, re-write exim configuration:
More info: http://www.exim.org/exim-html-4.40/doc/html/spec_13.html#SECT13.5