How do I configure the /etc/mailhelo file?

Question

When formatting the /etc/mailhelo file, what syntax should I use?

Answer

The /etc/mailhelo file contains Exim’s configuration for the HELO command, which initiates dialog between a mail server and a client. This file governs which domain sends the HELO command.

Use your preferred text editor to create an /etc/mailhelo file that resembles the following example:

example.com: example.com
sub.example.com: example.com
example.net: example.net
addon.example.net: example.net
*: hostname.example.com

Source: How to Configure the Exim Outgoing IP Address