+ STARTTLS if the server supports STARTTLS, and nil otherwise.
+
+:always-query-capabilities says whether to query the server for
+ capabilities, even if we're doing a `plain' network connection.
+
+:client-certificate should either be a list where the first
+ element is the certificate key file name, and the second
+ element is the certificate file name itself, or `t', which
+ means that `auth-source' will be queried for the key and the
+ certificate. This parameter will only be used when doing TLS
+ or STARTTLS connections.
+
+:use-starttls-if-possible is a boolean that says to do opportunistic
+STARTTLS upgrades even if Emacs doesn't have built-in TLS functionality.
+
+:nowait is a boolean that says the connection should be made
+ asynchronously, if possible."