Using Postfix @ localhost - cannot verify delivery server

Hope someone can help - I'm running Ubuntu 16.04LTS server with ISPConfig 3.1 as per this howtoforge guide:

I can set up the Delivery Server as an SMTP connection no problems - but when I try set it up as a TCP Stream, Mailwizz hangs during the POST and gives no hint as to what is actually happening/happened.

In the mail log, all I get is:
postfix/smtpd[00000]: connect from[]

I can use a webmail script (roundcube) to send mail and it works fine - I can also send mail using the CLI program "mail".

Anyone have a clue what I'm doing wrong?

@Elliot Cater - The TCP Stream server is a special type which works only with a certain software which i wrote and which unfortunately it's not maintained anymore. It will not work with SMTP, that is the reason why it does not work for you...
Ah ok - I assumed the TCP Stream setting was for putting mail directly into a local MTA's queue. I was under the impression that direct connection to the MTA (in my case postfix) would be the quickest way to send as there is no authentication necessary. Looking into it there is a Sendmail or PHP Mail function.

After reading this: - I've come to the conclusion that they recommend using SMTP with authentication - they seem to believe this offers the greatest feedback potential. I'm just a little worried that the server resources are going to be needlessly eaten up with the authentication process for every single piece of mail being sent.

Can you explain to me why SMTP has better feedback than localhost Sendmail or PHP mail?

@Elliot Cater - As i have explained via email(sorry for copy/paste but it's same deal), when using sendmail, PHP will spawn a new process each time which might have a considerable overhead than a network connection.
Also if you are using version >= you have an option in the delivery server settings (for smtp) to send multiple email messages per connection, so you could try that as well.