imap-ssl-program

imap-ssl-program is a variable defined in `imap.el'.
Its value is

("openssl s_client -quiet -ssl3 -connect %s:%p" "openssl s_client -quiet -ssl2 -connect %s:%p" "s_client -quiet -ssl3 -connect %s:%p" "s_client -quiet -ssl2 -connect %s:%p")

• This variable may be risky if used as a file-local variable.

Documentation:
A string, or list of strings, containing commands for SSL connections.
Within a string, %s is replaced with the server address and %p with
port number on server. The program should accept IMAP commands on
stdin and return responses to stdout. Each entry in the list is tried
until a successful connection is made.

You can customize this variable.