I have a Debian Stretch 9 installation which was happily delivering mail, via fetchmail/maildrop, to Dovecot. Then email can be accessed via LDAP in Maildir format.
When I installed Virtualmin over this existing system, the delivery format mysteriously changes to mbox!
Yes, I know this (existing system) is not the recommended way to do things, but I have a relatively clean Debian 9 installation and minimal special configurations. To clarify further, I do not have any virtual servers, or users, set up on this test system yet. However, the mail format is simply changed, out of the box. I installed using the install.sh script and this went very smoothly (apart from the mbox issue).
I have "mail_system=4" set everywhere (that I can find this configuration setting under usermin/webmin), and also for Dovecot: "mail_location = maildir:~/Maildir".
Mail is still delivered in the erroneous mbox format".
Thanks in advance for any helpful suggestions.