This tutorial assumes you have a paid Atlas account which includes the option to add custom subdomains. Get Atlas Here
Login to your cPanel account and go to Domains -> Zone Editor
Choose A RECORD and put a subdomain you would like to use for Atlas in Name. This can be anything like ex: email, inbox, atlas (best not to use "webmail" or "mail" because that's already in use by cPanel)
In Address add the Atlas IP 184.108.40.206 and click the ADD AN A RECORD button.
To double check the A record was added correctly, click MANAGE and scroll to the bottom of the page, you should see your new A record there. In our example this is email.atlasinbox.com (replace "atlasinbox.com" with your domain).
Now we need to setup the subdomain in the Atlas console, so login to your account at Roundcubeplus.com.
LOGIN HERE (Opens new tab)
After signing in you will be taken to the customer area click the Atlas Control Panel link.
Next head to the Domains section.
Here you will add the subdomain. Choose Use your own domain from the dropdown. Then make sure you use the same subdomain you just created in your cPanel, in our case this is email.atlasinbox.com. This will become the link to access your webmail.
Now setup your IMAP and SMTP so you can receive and send mail. This is usually formatted as “mail” followed by your domain name like this: mail.yourdomain.com for both IMAP and SMTP.
Now back in the Atlas Dashboard where you should see your cool new webmail domain. Click that domain to access your webmail.
From here on out you no longer need to go through any of the steps above. Just bookmark that subdomain and access your email from there.
Now that you have webmail setup you can login to any email accounts on that server.
The webmail link will work on desktops, mobiles and tablets.