There is nothing wrong with our mail services, it's ossn bypassing the sendmail service and sending email directly so getting bounced.
I don't see how this is outside of ossn? A lot of people would not understand this and definitely not understand that their host is sending email and it's all getting bounced because no valid mail server is sending.

Anyhow, how can I change this behavior? I need ossn to send using php's sendmail directives or even sendmail directly which is configured to relay.

Dear Mick, we can not include everything in settings and this is outside control of OSSN. You must configure your MTA to send mails correctly as OSSN have no control.

BTW, signing up to this site is also a hit and miss. I've had two people sign up and both have never seen the activation email. Presumably, because the host is sending directly and is not a proper mail server nor with PTR.

Just an assumption since I also had to add the sending IP and domain to the mail server hosts file otherwise, never received my activation for this site.

Anyhow, that smtp component is useless for me as far as I can see.
I need to get rid of this phpmailer and have emails sent by sendmail.
Sendmail is already configured to relay to one of our mail servers.

Does anyone know how to do that?

The reason I was upset to learn about this is because of the constant, non stop work it takes to prevent mail servers from getting blacklisted. It's the last thing someone wants because it can take a crazy amount of hours to fix the problem. Sometimes, you cannot even do anything about it, you have to wait and never know how long that will be.

This is something I suggest be included in settings, not as an afterthought.

As nothing is mentioned anywhere about this, I didn't realize the host was sending email directly to remote mail servers. That's a ridiculous default setting and should be a GUI setting that can be clearly set up.

In our case, we have a mail server on the LAN and all hosts use sendmail to relay to that host so that all email goes out over the Internet from a real, valid email server. Until I found this, I didn't realize the host itself is sending out which is harming the domain.

Why isn't there a full out manual for this and if there is, I've yet to find it.