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Fri, Oct 05, 2018 9:39 pm

Checking port 465 connectivity

I needed to check on whether a system was functioning as a mail server listening on the Simple Mail Transfer Protocol Secure (SMTPS) port, TCP port 465. One way to check is to attempt to establish a telnet connection to that port. E.g.:

$ telnet 192.168.248.91 465
Trying 192.168.248.91...
Connected to esa.example.com
Escape character is '^]'.
^C
^CConnection closed by foreign host.
$

If you see a "connected to" message that indicates the system is listening on that port and you can terminate the connection by hitting Ctrl-C a couple of times. But you can also use an OpenSSL command as shown below to test SMTPS connectivity on TCP port 465:

$ openssl s_client -connect 192.168.248.91:465 -quiet
depth=0 /OU=Zimbra Collaboration Server/CN=esa.example.com
verify error:num=20:unable to get local issuer certificate
verify return:1
depth=0 /OU=Zimbra Collaboration Server/CN=esa.example.com
verify error:num=27:certificate not trusted
verify return:1
depth=0 /OU=Zimbra Collaboration Server/CN=esa.example.com
verify error:num=21:unable to verify the first certificate
verify return:1
220 esa.example.com ESMTP Postfix
quit
221 2.0.0 Bye
$

If you leave the -quiet off the end of the command, you can see additional information about the certificate on the email server.

Related articles:

  1. Using the openssl command to troubleshoot POP3S

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