If the following error message appears in mail.log:
fatal: main.cf configuration error: virtual_mailbox_limit is smaller than message_size_limit
it is a misconfiguration.
In the file
/etc/postfix/main.cf
file, the following entry must be added:
virtual_mailbox_limit = 0
After restarting Postfix, the problem should be solved:
$ /etc/init.d/postfix restart