imap-stream-alist

imap-stream-alist is a variable defined in `imap.el'.
Its value is

((gssapi imap-gssapi-stream-p imap-gssapi-open)
 (kerberos4 imap-kerberos4-stream-p imap-kerberos4-open)
 (tls imap-tls-p imap-tls-open)
 (ssl imap-ssl-p imap-ssl-open)
 (network imap-network-p imap-network-open)
 (shell imap-shell-p imap-shell-open)
 (starttls imap-starttls-p imap-starttls-open))

Documentation:
Definition of network streams.

(NAME CHECK OPEN)

NAME names the stream, CHECK is a function returning non-nil if the
server support the stream and OPEN is a function for opening the
stream.