Why is the mailbox size different than the Mail statistics?

Question

Why is the Domains > example.com > Mail addresses mailbox size different than the Subscription > example.com > View more statistics > Mail statistics?

Answer

These are 2 different types of calculation:

1. Mailbox size in Domains > example.com > Mail addresses corresponds to the size of the mailbox of the following directory. 

   # du -sh /var/qmail/mailnames/example.com
   257G /var/qmail/mailnames/example.com

   Mailing lists are not included in /var/qmail/mailnames/example.com/<mailboxname>/Maildir/maildirsize:

   # du -sh /var/lib/mailman/archives/*
   907G /var/lib/mailman/archives/private

2. The Mail statistics in Subscription > example.com > View more statistics is a sum of the mailboxes + the mail lists. This value can be verified in bytes with the following query for subscription example.com:

   # plesk db
   MariaDB [psa]> SELECT SUM(mailboxes) + SUM(maillists) AS mail, 0 FROM disk_usage AS disk_usage INNER JOIN domains AS domains ON domains.id = disk_usage.dom_id WHERE (dom_id =(select id from domains where name="example.com"));
   +---------+---+
   | mail | 0 |
   +---------+---+
   | 12288 | 0 |
   +---------+---+
   1 row in set (0.00 sec)