I'm trying to use this command to check on port 587 for my postfix server.

Using nmap -P0 mail.server.com I see this:

Starting Nmap 5.51 ( http://nmap.org ) at 2013-11-04 05:01 PST
Nmap scan report for mail.server.com (xx.xx.xx.xx)
Host is up (0.0016s latency).
rDNS record for xx.xx.xx.xx: another.server.com
Not shown: 990 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
25/tcp   open  smtp
110/tcp  open  pop3
111/tcp  open  rpcbind
143/tcp  open  imap
465/tcp  open  smtps
587/tcp  open  submission
993/tcp  open  imaps
995/tcp  open  pop3s
5666/tcp open  nrpe

So I know the relevant ports for smtps (465 or 587) are open.

When I use openssl s_client -connect mail.server.com:587 -starttls smtp I get a connection with all the various SSL info. (Same for port 465).

But when I try libexec/check_ssmtp -H mail.server.com -p587 I get:

CRITICAL - Cannot make SSL connection.
140200102082408:error:140770FC:SSL routines:SSL23_GET_SERVER_HELLO:unknown protocol:s23_clnt.c:699:

What am I doing wrong?