A response code consists of data inside square brackets in the form of an atom,
possibly followed by a space and arguments. The response code
contains additional information or status codes for client software
beyond the OK/NO/BAD condition, and are defined when there is a
specific action that a client can take based upon the additional
information.
7.5. Server Responses - Command Continuation Request
7.2 - 7.4 Server and Mailbox Status; Mailbox Size; Message Status
IMAP4rev1 defines three possible greetings at connection startup.
7.1. Server Responses - Status Responses