Private integrated with user s computer. Owned and operated by a company for its own internal messaging requirements. Public systems you must pay for number and length of messages sent and stored. Private systems offer better integration with customer-owned systems, but public systems offer a wider range of delivery options and more users. Single- users of a shared computer system can exchange messages.
Users have identifiers, mailboxes, mail facility is an application program.
Limitation messages can only be exchanged among users of that particular system. Electronic mail functions are grouped into user agent and message transfer agent. User agent visible to the user. Include facilities for preparing and submitting messages for routing to destinations and utility functions to assist the user in filing, retrieving, replying, and forwarding. Message transfer agent - user doesn't interact directly with it.