You can check the list of your partitions with this command:

df -h

Before moving the emails you should first check the actual usage of /var/vmail with this command

du -hs /var/vmail

Step 1: We advise you stop postfix and dovecot during this process

systemctl stop postfix dovecot

Step 2: copy the vmail folder to the new location

rsync -av /var/vmail/ /home/vmail/

The Above command will copy the folder vmail to /home, so we will have /home/vmail. All permissions will be maintained as well.

Step 3: Backup /var/vmail folder

mv /var/vmail /var/vamil.orig

Step 4: create a symlink from the new location to old location

ln -s /home/vmail /var/vmail

Step 5: start postfix and dovecot

 systemctl start postfix dovecot

You can then test send and receipt of emails. If all is ok, then you can delete the /var/vmail.orig folder to make the space available.

Correct permissions are required (edit if you use different location)
chmod 770 /var/vmail
chown vmail.mail /var/vmail

MAIL DISK QUOTA (postfix vmail)
Please note that if you want quota to work for mail then it needs to be on the same partition as user home files.
If you have a custom partition for /home then you will need to have /var/vmail moved per instructions to /home/vmail

CWP SECURE KERNEL
CWP Secure Kernel will allow only /var/vmail and /home/vmail locations, other locations might have issues with quota.