[RCU] [SMTP, SSL] Roundcube 1.2.2 - never use TLSv1.2 and always fallback to TLSv1 when sends mail

Hello,

Roundcube always use TLSv1 for sending mail (used on same host as mailserver with both RSA and ECDSA certs, ports 25 and 587). When I disable all TLS/SSL versions except TLSv1.2 in Postfix config, I cannot send mails from Roundcube (SMTP Error: Authentication failure: STARTTLS failed (Code: ) in /var/www/roundcube/program/lib/Roundcube/rcube.php on line 1649 (POST /?_task=mail&_unlock=loading1478615054443&_lang=en&_framed=1&_action=send) ) but can from desktop/mobile client. Connection from Roundcube to IMAP server is always use TLSv1.2, external servers (i.e. GMail) connect with TLSv1.2.

What could cause this behaviour? Is it possible to force TLSv1.2 for SMTP?