We no longer provide the Atmail plug-in, because its developer decided to stop providing the open-source version of their product. This was the first in the list of search results when I entered "atmail" in the search box in the upper right corner of every page on this site:
It's hardly cPanel's fault that the developer stopped making the product, is it? cPanel do provide 3 other webmail clients for you to use. They did also announce that this was going to happen last year:
Or, I believe cPanel provide a way for you to incorporate custom webmail apps. So you may be able to purchase Atmail yourself and install it on your own server:
We still include Roundcube, Horde and Squirrelmail in cPanel. All three Webmail clients log into your mailbox just like Atmail. They look and work differently, but log into the mailbox the same way (IMAP) and show you the same data.