[Dovecot] change initial permissions on creation of mail folder

Installed dovcot from Debian .deb file. Creating a new account for system users sets permission for user-only. Where to change initial permissions on creation of mail folder and other subdirectories.

Installed dovecot using "apt-get install dovecot-imapd dovecot-pop3d". Any time when I create a new account in my mail client for a system user, Dovecot tries to create ~/mail/.imap/INBOX. The permissions for mail and .imap are set to 0700. By this permissions INBOX can not be created leading to an error message in log files. When I manualy change the permissions to 0770, INBOX is created