SIMS allows simultaneous IMAP and POP client connections. While this is standard for most IMAP clients, for POP clients this means that if a connection is dropped, or a user wants more than one connection, then a user can immediately reconnect and have access to her messages without having to wait for the POP server to time-out.
When multiple IMAP clients connect to a mailbox, all flag changes (new, seen, deleted, answered, and so on) and message states (new mail, removed message) are shared between clients by means of the IMAP protocol.
When IMAP and POP clients connect to a mailbox, all flag changes (new, seen, deleted, answered, and so on) and message states (new mail, remove message) are shared between IMAP clients. POP clients, however, will not see IMAP deleted messages. When a POP client connects to a mailbox, it simply grabs a snapshot of the mailbox which persists until the POP session is terminated.
When multiple POP clients connect to a mailbox, each grabs a unique snapshot of the mailbox state at connect time. Each connection's unique snapshot persists until the POP session is terminated. Any changes to a mailbox's state are seen by clients that connect after these changes are made.