Ssl – Openssl: Extract root certificate from certificate chain

You don't need the root certificate in the chain (though I don't believe having it hurts anything).

That error is more of a warning from openssl in this case I think. I believe what it means is just that openssl doesn't know that it should trust the root certificate of that chain. If you pull out just the root certificate from that bundle and point that openssl command at it with the -CAfile argument I expect that "error" should go away.

Inside the sf_bundle.crt file you should see two

-----BEGIN CERTIFICATE-----
....
-----END CERTIFICATE-----

blocks (possibly with plain text above each block showing what certificate the block contains). If you split each of those blocks into its own file so you end up with block1.crt and block2.crt you should be able to run openssl x509 -noout -subject -in <file.crt> on each of them to get their respective certificate subject lines.

Assuming that block2.crt has a subject line of /C=US/O=Starfield Technologies, Inc./OU=Starfield Class 2 Certification Authority you should then be able to run openssl s_client -CAfile block2.crt -connect imap.domain.ltd:465 and that should, hopefully, connect without giving you the self-signed certificate error.

Be sure to include all the intermediate certificates and see that they are up to date. If test.crt is in fact a file containing only /C=SE/O=AddTrust AB/OU=AddTrust External TTP Network/CN=AddTrust External CA Root, this is the correct approach. You can include the root as well, and most clients will accept such chains, but a few will choke.

In general it's best practice to include all the certificates from yours to the last one before the root, in case a client doesn't have the last intermediate.

It's also possible that test.crt contains things other than the correct chain. OpenSSL doesn't do partial chain validation by default (in older versions, it doesn't do it at all). When operating in this mode it doesn't care what is in /etc/ssl/certs.

Alternatively, you may be presenting an expired intermediary certificate. CAs often recertify their intermediates with the same key; if they do that, just download the updated intermediate CA certificate and replace the expired one in your chain.

Finally, with openssl s_client, you need to specify what it is validating against. For example, use the option -CApath /etc/ssl/certs or -CAfile your_ca.crt. For the first option use your system's trust store, and for the second option specify the root CA certificate.