This only works only if you have a dedicated IP on your cPanel account
Many people are having problems accessing cPanel mail clients , entering mail client from cPanel takes a lot of times and memorizing the URL which is located on “http://domain.com:2095/3rdparty/roundcube/index.php” is not easy, also it is very long.
after some research, I found an excellent solution to solve this problem one time forever and for all addon domains !
cPanel makes a CNAME entry for mail.maindomain.com and all mail.addondomains.com to maindomain.com automatically , we will use it to do the trick.
create .htaccess file in the root directory of your main domain ( /public_html folder ) and copy and paste the following code into it :
Options +FollowSymlinks
RewriteEngine on
RewriteCond %{http_host} ^mail\.(.*)\.com$ [NC]
RewriteRule ^(.*)$ http://%1.com:2095/3rdparty/roundcube/index.php [R=301,NC,L]
it will redirect mail.maindomain.com and all of mail.addondomains.com to http://domain.com:2095/3rdparty/roundcube/index.php