Issue the following command:

openssl s_client -CApath /etc/ssl/certs/ -connect <host.domian.tld>:993

For testing on a mail server supporting both non-encrypted and encrypted (TLS) connections using STARTTLS method:

openssl s_client -CApath /etc/ssl/certs/ -starttls smtp -connect <host.domian.tld>:25

There should be stated quite at end of command output:

    Verify return code: 0 (ok)

before an eventual greeting message of the server.

A bit above, you can check the certificate chain completeness:

Certificate chain
 0 s:/description=3UwjnK9kRZ2wUo8e/C=CH/CN=domain1.ownspace.ch/emailAddress=hostmaster@ownspace.ch
   i:/C=IL/O=StartCom Ltd./OU=Secure Digital Certificate Signing/CN=StartCom Class 1 Primary Intermediate Server CA
 1 s:/C=IL/O=StartCom Ltd./OU=Secure Digital Certificate Signing/CN=StartCom Class 1 Primary Intermediate Server CA
   i:/C=IL/O=StartCom Ltd./OU=Secure Digital Certificate Signing/CN=StartCom Certification Authority
---

The last i(ssuer) is the root cert that most client will trust.