[Dovecot] permissions for dbox attachment directory

I am running dovecot 2.0.13 on CentOS5, set up with only local system users, and single instance store set up in a single directory.

e.g. mail_attachment_dir = /var/mail/attachments

I get errors when importing mail to IMAP because the attachment directories are owned by the respective mail account user(s) and so cannot be accessed or written to by other users.

Are there some user id setting I need to change, or do I have to run dovecot with virtual users under a single system account?

Thanks!