The syntax of the IMAP protocol is strictly defined, both the client and the server is either reading a complete line or a sequence of octets prefixed with the length of the sequence.

Request lines start with the tag, followed by a command verb identifying the operation. Each command might have one or more arguments separated by spaces. Each argument has an associated type, one of: ATOM, LITERAL, STRING, LIST. The type further specifies the syntax how these arguments are represented.

A response from the server might be sent directly in response to a request, or unilaterally whenever the server implementation feels it appropriate. The response includes a response verb with zero or more arguments. Note that there might be more response verbs returned for a single command and the response verbs have no direct relationship with the request verb.

Content (e.g.: mail bodies) are transferred as literals embedded in commands and responses. There is no separate bulk transfer mode in the protocol like in POP3 or SMTP. This results in extremely large request/response sizes.

Each message might have one or more associated message flags like '\Deleted' or '\Seen'.