system library:BIO_connect:Connection refused:../crypto/bio/bio_sock2.c

I got the strange error message 

system library:BIO_connect:Connection refused:../crypto/bio/bio_sock2.c

on executing 

openssl s_client -starttls smtp -connect mail.area23.at:587

when testing sendmail smtp tls with an Let's Encrypt SSL certificate.

If you got the same error message, ensure that

1. sendmail is running (on mail server / relay host)
   test it with init rc script 
     /etc/init.d/sendmail status
   or simple by process listing tools, like 
     pstree | grep sendmaail
     pidof sendmail-mta
     ps fauxwww | grep sendmail
   and network server socket is bound 
     netstat -avn 2>&1 |less
   or fuser and your sendmail port
     fuser -v -n tcp 25
   or various variants of lsof, like
     lsof -i TCP:25
   
   
2. sendmail is well configured (on mail server / relay host)
   this will take a little bit too long for that article
   in my case it was a wrong IPv6 address in sendmail.mc
   DAEMON_OPTIONS(`Family=inet6, Name=MTA-v6, Port=smtp, Addr=2600:1f14:1d6d:f202:8d53:fc57:c45d:6590')dnl
    
3. mail server is reachable from outside 
   test it with 
    telnet [mailserver] [port]
   or brutal way with a network scanner, like nmap
     nmap -p 25,465,587 [mailserver]
   
   if mailserver is not reachable from outside, 
   then you must lookup on server and on client firewall rules, e.g.
     iptables -t filter --list
     iptables -6 -t filter --list

You could also simply run strace to see, what happens:
  strace openssl s_client -starttls smtp -connect mail.area23.at:587