Based on what I've seen so far, it sounds like you're talking "in the clear" when you need to be on an ssl/tls channel instead. So, instead of:
you would want to try:
$ telnet outbound.mailhop.org 465
Then use your HELO/EHLO to get a response from the server. But then you need to make sure postfix is talking smtps instead of smtp (and then wanting to STARTTLS). Here are some useful references for this:
$ openssl s_client -connect outbound.mailhop.org:465
Via that first link, you'll find the following statement (which leads you toward the second link provided above):
So if you do this, you'll setup stunnel as described in the second link and then tell postfix to route email through that tunnel.
The Postfix SMTP client does not support the obsolete "wrappermode" protocol, which uses TCP port 465 on the SMTP server. See TLS_README for a solution that uses the stunnel command.