In fact it is the php limit which reach the 2MB…I did some work in the past on roundcunemail and indeed some php admin value should be added to set the limit under 10MB but only in the roundcubemail virtualhost.
Of course we can add a gui for roundcubemail…but if all module gets a gui…You will have quick too much menus in your server manager. Maybe a db value should be better.
In order to solve your issue you can install nethserver-php-scl
If you want a gui to set the limit values of php or you can use the cli command to set the value of php by hand in your terminal.