Unable to send message from Roundcube: SMTP Error (-1): Connection to server failed

Applicable to:

The following error pops up after logging into Roundcube WebMail:

CONFIG_TEXT: Connection to storage server failed
Unable to send a message using Roundcube webmail with the following error:

PLESK_INFO: SMTP Error (-1): Connection to server failed
The entries like below can be found in /var/log/maillog:

CONFIG_TEXT: admin 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=
The error below is found in the /var/log/plesk-roundcube/errors:

CONFIG_TEXT: 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)
While checking the certificate on mail server (replace example.com with the valid domain name or hostname), it can be seen that certificate expired:

# echo | openssl s_client -connect example.com:465 -servername example.com 2>/dev/null | grep 'Verify return code'
Verify return code: 10 (certificate has expired)

Mail server SSL/TLS certificate expired.

It is required to renew the certificate for Mail Server.

For that, log into Plesk and renew the certificate with one of the following approaches:

If Let's Encrypt certificate is used, follow the instructions from the article How to secure mail server with Let's Encrypt certificate?
If a purchased certificate is used, follow the instructions from the article How to secure a mail server with an SSL certificate?