/var/log/roundcubemail/errors.log
Code:
[18-May-2016 23:59:29 +0200]: <e06dj749> SMTP Error: Connection failed: Failed to connect socket: Connection timed out in /usr/share/roundcubemail/program/lib/Roundcube
/rcube.php on line 1712 (POST /roundcube/?_task=mail&_unlock=loading1463609608645&_lang=en_US&_framed=1?_task=mail&_action=send)