5675507 - Message storage and delivery system (voicemail)
6208638 - Method and apparatus for transmission and retrieval of facsimile and
            audio messages over a circuit or packet switched network (spandsp sending faxes via email)
6350066 - Systems and methods for storing, delivering, and managing messages (think vmail.cgi)
6564321 - Systems and methods for storing, delivering, and managing messages (again vmail.cgi)
6857074 - Systems and methods for storing, delivering, and managing messages (same as the other two)
6597688 - Scalable architecture for transmission of messages over a network (usage with spandsp)

Non-Asterisk Related:
6073165 -Filtering computer network messages directed to a user's e-mail box based on user defined filters, and forwarding a filtered message to the user's receiver