[Cryptography] How to get certificates on email server?

[John Denker]

On 04/18/2016 10:52 AM, hbaker1 wrote:

> Stupid question: I'm getting a certificate error when attempting to
> send email.  My email program says that the certification chain can't
> be verified.
> 
> Before I accept this dubious certificate, I'd like to look up this
> server's certificate myself & check it by hand.
> 
> How to do this?  (Either Windows or Linux is fine, since I'm not
> trying to actually send mail; just check the certificate.)
 
 :| openssl s_client -connect av8n.com:465

Or for even more detail:

 :| openssl s_client -debug -tlsextdebug -showcerts -connect av8n.com:465

On some old buggy versions the following works better:

 :| openssl s_client -CApath /etc/ssl/certs -connect av8n.com:465


Port numbers of interest include
  urd             465/tcp         ssmtp smtps     # URL Rendesvous Directory for SSM
  submission      587/tcp                         # Submission [RFC4409]
  https           443/tcp                         # http protocol over TLS/SSL