The du command can be used to determine the size of the /var/spool/clientmqueue directory, such as 6.4 GB in this example.

~]# du -hs /var/spool/*
6.4G    /var/spool/clientmqueue

The /var/spool/clientmqueue directory is where emails that are pending to be sent are stored. The /var/spool/clientmqueue directory often grows quite large if the sendmail service is stopped and disabled.

~]$ sudo systemctl status sendmail
○ sendmail.service - Sendmail Mail Transport Agent
     Loaded: loaded (/usr/lib/systemd/system/sendmail.service; disabled; vendor preset: disabled)
     Active: inactive (dead)

Simply starting and enabling the sendmail service may cleanup the /var/spool/clientmqueue directory.

sudo systemctl start sendmail
sudo systemctl enable sendmail