RFC 2828 (Internet Security Glossary) defines the Internet Message Access Protocol, version 4 (IMAP4) as an Internet protocol by which a client workstation can dynamically access a mailbox on a server host to manipulate and retrieve mail messages that the server has received and is holding for the client.
IMAP4 has mechanisms for optionally authenticating a client to a server and providing other security services.
MIME is the MultiPurpose Internet Mail Extension. MIME extends the format of Internet mail to allow non-US-ASCII textual messages, non-textual messages, multipart message bodies, and non-US-ASCII information in message headers.
Simple Mail Transfer Protocol (SMTP) is a TCP-based, application-layer, Internet Standard protocol for moving electronic mail messages from one computer to another.
Privacy Enhanced Mail (PEM) is an Internet protocol to provide data confidentiality, data integrity, and data origin authentication for electronic mail.
Source: SHIREY, Robert W., RFC2828: Internet Security Glossary, may 2000.