Following actions can be performed if /usr is consuming high disk usage:
Check whether it is getting filled up due to mailman.
root@server[/usr]# du -sch /usr/local/cpanel/3rdparty/mailman
4.8G /usr/local/cpanel/3rdparty/mailman
4.8G total
If mailman is having maximum disk usage, you can move it to /home and create a symblolic link to the original location.
Firstly stop cPanel and take a backup of mailman before proceeding with the task.
cp -R /usr/local/cpanel/3rdparty/mailman /home/mailman.bak
After doing that, move only these directories in /usr/local/cpanel/3rdparty/mailman/ location:
archives
lists
logs
messages
After that, symlink them back to /usr/local/cpanel/3rdparty/mailman location:
ln -s /home/mailman/archives /usr/local/cpanel/3rdparty/mailman/archives
ln -s /home/mailman/lists /usr/local/cpanel/3rdparty/mailman/lists
ln -s /home/mailman/logs /usr/local/cpanel/3rdparty/mailman/logs
ln -s /home/mailman/messages /usr/local/cpanel/3rdparty/mailman/messages
Finally, chown for ownership to mailman:mailman:
chown -R mailman:mailman /usr/local/cpanel/3rdparty/mailman/{archives,lists,logs,messages}
Start cPanel once the work is complete and check the websites and mail and ensure whether everything is working fine.