The cPanel interface reports a greater value than "du" for the space used by an email account

Symptoms

Running "du" gets you a smaller amount for email than the cPanel interface is reporting.  This difference can be significant.

Description 

This is caused by the setting "Compress Messages" being selected in WHM Home » Service Configuration » Mailserver Configuration.  When this is set, the size of the messages on disk is a smaller, compressed size, while the amount reported by cPanel remains the full, uncompressed size.

Workaround

Use the API to determine how much space cPanel will report for the email account:

uapi --user=username Email get_disk_usage user=user domain=example.com