For my configuration to work, I have to use ssl:// for IMAP and tls:// for SMTP:

$config['default_host'] = 'ssl://shrub.ca';
$config['default_port'] = 993;

$config['smtp_server'] = 'tls://shrub.ca';
$config['smtp_port'] = 587;

(1) Why is that?

(2) Would/can you ever use tls:// with IMAP? In which cases?

(3) Why would SMTP fail with ssl://?

ERROR: fsockopen(): unable to connect to ssl://localhost:587 (Unknown error) (0)
ERROR: Failed to connect socket: fsockopen(): unable to connect to ssl://localhost:587 (Unknown error) ()
<5kagdbq1> SMTP Error: Connection failed: Failed to connect socket: fsockopen(): unable to connect to ssl://localhost:587 (Unknown error) in /usr/local/roundcube/program/lib/Roundcube/rcube.php on line 1587 (POST /?_task=mail&_unlock=loading1450230927143&_lang=en_US&_framed=1?_task=mail&_action=send)

ERROR: fsockopen(): unable to connect to ssl://shrub.ca:587 (Unknown error) (0)
ERROR: Failed to connect socket: fsockopen(): unable to connect to ssl://shrub.ca:587 (Unknown error) ()
<5kagdbq1> SMTP Error: Connection failed: Failed to connect socket: fsockopen(): unable to connect to ssl://shrub.ca:587 (Unknown error) in /usr/local/roundcube/program/lib/Roundcube/rcube.php on line 1587 (POST /?_task=mail&_unlock=loading1450230965840&_lang=en_US&_framed=1?_task=mail&_action=send)

Paul