fsockopen on SSL does not working on specific host

On working production server, at some point (June 24 at 13:18), fsockopen started to fail with no seen reason.

$sock = fsockopen("ssl://mail.dupcheck.org",993, $errno, $errstr, 300);

it returns: $sock = false, errno = 0, errstr = "" quite fast.
it opens other servers.

Important to mention,

it does not work both, on linux web server and on windows (WAMP) development server
MS Outlook and other mail clients perfectly connect to this server and fetch the mail.
that day (June 24) we had some issues with GoDaddy SSL certificate - it started to show improper date! We called GoDaddy's support then and they say, the issue is fixed.
Since then, for almost a month, SOMETIMES fsockopen worked and we received emails, and nobody paid attention, nobody checked that specific mailsend log ... until today it reached 300M!

====

PHP 7.3.3-1, ubuntu16.04.1

Yes, I read manuals, googled for several hours, no relief. Nobody knows what's that, including system administrators.

Solution

From the comment:

... openssl s_client perfectly working ...

It works in that it connects and does a TLS handshake. But it does not work perfectly since it clearly shows that the certificate validation fails:

$ openssl s_client -connect mail.dupcheck.org:993
...
Certificate chain
 0 s:/OU=Domain Control Validated/CN=mail.dupcheck.org
   i:/C=US/ST=Arizona/L=Scottsdale/O=GoDaddy.com, Inc./OU=http://certs.godaddy.com/repository//CN=Go Daddy Secure Certificate Authority - G2
...
    Verify return code: 10 (certificate has expired)
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Taking a closer look at the certificate:

    Serial Number:
        92:a0:2d:2a:2f:28:6a:d5
Signature Algorithm: sha256WithRSAEncryption
    Issuer: C=US, ST=Arizona, L=Scottsdale, O=GoDaddy.com, Inc., OU=http://certs.godaddy.com/repository/, CN=Go Daddy Secure Certificate Authority - G2
    Validity
        Not Before: Jun 24 10:16:38 2016 GMT
        Not After : Jun 24 10:16:38 2019 GMT

Thus, this certificate was expired some weeks ago. This is likely the reason the connection fails.

... tls.imirhil.fr/tls/mail.dupcheck.org:993 does not seem to show anything extraordinary.

It looks like this site does not check the validity of the certificate at all.