Please have a look at /var/log/maillog, and see if you can find any relevant log message.
Then find the maildirsize file of your user and paste here its contents
find /var/lib/nethserver/vmail/ -name maildirsize
...
cat /var/lib/nethserver/vmail/USERNAME/Maildir/maildirsize
Just to have an idea of mailbox disk size (should not be exactly the same of mail quota)
du -h -s /var/lib/nethserver/vmail/USERNAME