When imip-agent is configured to use LMTP, it connects directly to mail storage solutions. By using LMTP from the agent software, the issue of configuring the mail system to integrate with such solutions is avoided, but then those solutions must expose their LMTP interface appropriately.
Although this topic is largely beyond the scope of this documentation, systems such as Cyrus and Dovecot can be configured to provide a Unix domain socket offering support for LMTP connections.
For Cyrus on Debian, the following bug report is pertinent:
A permanent change in permissions on the Cyrus LMTP socket may be required to make delivery available to the lmtp group:
dpkg-statoverride --force --update --add cyrus lmtp 750 /var/run/cyrus/socket
See the system users documentation for a discussion of users and groups.