The Internet Message Access Protocol (IMAP) version 4 is specified by RFC 3501. This specification is sometimes also called as ‘IMAP4 rev1’ because it is a revision of the initial form of the IMAP4 protocol specified in RFC 2060 and RFC 1730.
IMAP4 is a sophisticated protocol intended for use with mailboxes on a server using a PC in the online (or offline) mode. At the same time, we can work with our mailboxes from several applications. Some applications even establish two TCP connections with the IMPA4 server (for example, MS Outlook)—one connection for working with mailboxes and the other one for working with individual items (email messages). An IMAP4 protocol server uses the well-known port 143/TCP.
When IMAP4 is working with a mailbox (during an established TCP connection), another application can change the content of this mailbox (for example, the SMTP server records newly received mail in the mailbox). These events (for example, a new message coming...