Can't send mail from Roundcube: SMTP Error (-1): Connection to server failed

Symptoms

• Opening Roundcube or trying to send an email with it ends up in error

  Connection to storage server failed

  SMTP Error (-1): Connection to server failed

• Entries about an expired certificate and SSL errors are logged to /var/log/maillog and /var/log/plesk-roundcube/errors

  dovecot: imap-login: Disconnected (no auth attempts in 0 secs): user=<>, rip=::1, lip=::1, TLS handshaking: SSL_accept() failed: error:14094415:SSL routines:SSL3_READ_BYTES:sslv3 alert certificate expired: SSL alert number 45, session=

  postfix/smtpd: warning: TLS library problem: error:14094415:SSL routines:SSL3_READ_BYTES:sslv3 alert certificate expired:s3_pkt.c:1275:SSL alert number 45:

  ERROR: stream_socket_client(): unable to connect to ssl://localhost:465 (Unknown error) (0)
  ERROR: Failed to connect socket: stream_socket_client(): unable to connect to ssl://localhost:465 (Unknown error) ()
  <fav2islm> SMTP Error: Connection failed: Failed to connect socket: stream_socket_client(): unable to connect to ssl://localhost:465 (Unknown error) in /usr/share/psa-roundcube/program/lib/Roundcube/rcube.php on line 1667 (POST /?_task=mail&_unlock=loading1533740030256&_lang=es&_framed=1&_action=send)

Cause

The mail server certificate expired.

Resolution

Manually renew the certificate for the mail server.