Use Exim's message_size_limit RoundCube's configuration
There are a few places that affect the notice in RoundCube that tells the webmail user how much they can upload. If the max_message_size is set in RoundCube configuration file, a percentage of that is shown. If it's not, the post_max_size from the php.ini file is used.
Currently, in cPanel's RoundCube setup, the max_message_size is not set in the config.inc.php file. RoundCube has its own php.ini file which sets the post_max_size using the Tweak Setting php_post_max_size. This can be confusing for webmail users because Exim's message_size_limit can be set to less than the minimum allowed for that Tweak Setting. This means that RoundCube says that they can upload a file larger than they're allowed to send.
This feature request asks for the max_message_size to be set in the config.inc.php for Roundcube. Ideally, it will be set to the value set for Exim's message_size_limit (default 50MB, not included in cPanel's default configuration), and be updated should the Exim Configuration Manager's Advanced Editor be used to update the message_size_limit.