I have found that the webmail.domain.tld redirect to domain.tld:2095 (the webmail port of cPanel) may not work as well.
So here's a simpler .htaccess rule which you can set at the root of a cPanel account so it can affect all addon domains and best of all, it does not have a hardcoded domain on it 
Code:
# Redirect webmail.domain.tld to domain:2095 for cPanel Webmail only
RewriteCond %{HTTP_HOST} ^webmail\.(.+)$ [NC]
RewriteRule ^(.*)$ http://%1/webmail/$1 [R=301,L]
This will make sure webmail.domain.tld is redirected to domain.tld/webmail which in turn cPanel will push to domain:2095 - aka your webmail login. 
Enjoy!