Just in case someone else has the same problem, I had to change the following two variables

mail_home = /mnt/home/imapd/%d/%n mail_location = dbox:/mnt/home/imapd/%d/%n

to mail_home = /mnt/home/imapd/%d/%n mail_location = dbox:~/

"When mail_location begins with%h or~/, its permissions are copied from the first existing parent directory if it has setgid-bit set. This isn't done when the path contains any other %variables."