Thanks for pointing me in the right direction. I've done what you told me to by first downloading mb2md and placing it in the /usr/bin directory. I then ran the following command:
I was logged in a root
sudo -u john mb2md-3.20.pl -s /var/mail/john -d /var/www/web3/user/john/Maildir
It seemed to have worked because I noticed that it created 3 additional folder in /var/www/web3/user/john/Maildir:
I then went into ISPConfig manager and check the box to use maildir and restarted ISPConfig. Unfortunately, when I sent a mail from the outside to John's account, it didn't come through. The log said it was delivered though. I then went back into ISPConfig manager and unchecked the box for maildir and restarted ISPConfig. I sent another mail and it came through.
I was wondering if I had to change all the mailboxes into maildir format before this could work. I have 3 additional mail account in my /var/mail/ folder. They are:
Do I have to convert those as well? If so, what would the destination directory be?