How to expose the URL via webmail server

Hello,
We can expose the URL via webmail server in following way  :-
Part-1

Stp-1: Log on in domain provider  account (i.e.  my-account-manager.com,nettigritty.com etc)
Stp-2: Open the manage page of the domain.
Stp-3: Link the URL with the exposed ip of webmail server.
Stp-4: save and exit from the page.

Part-2

Stp:-1 Now log in in local webmail server.
Stp:-2 cd /opt/lamp/etc/exposedhost 
Stp:-3 make a new file with the name of exposed URL.
Stp:-4 Add following contains in this file;

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