SMTP STARTTLS certificate negotitiation via telnet

Question

I am trying to start tls in sendmail, but I do not know how to use certificate. Please suggest me way

> telnet localhost 25 Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. 220 <machinename> ESMTP Sendmail <version>; <date>;localhost(OK)-localhost [127.0.0.1] EHLO localhost 250-<mahinename> Hello localhost [127.0.0.1], pleased to meet you 250-ENHANCEDSTATUSCODES 250-PIPELINING 250-EXPN 250-VERB 250-8BITMIME 250-SIZE 250-DSN 250-ETRN 250-AUTH DIGEST-MD5 CRAM-MD5 250-STARTTLS 250-DELIVERBY 250 HELP STARTTLS 220 2.0.0 Ready to start TLS 

When and How should I use/provide the certificate?

Answer 1

You can't, because as soon as you start using TLS, the conversation becomes encrypted, and you probably don't speak that language ;)

Here is what you can do instead:

openssl s_client -debug -starttls smtp -crlf -connect localhost:25 

OpenSSL will do the STARTTLS handshake for you and you will be able to pick up the conversation from there (decrypted automatically on the fly).