Sending emails via WP

I have a WP site and am using the WP Mail SMTP plugin to send emails for manage our subscriber list for sending notifications when a new post is made.

About 1 in 4 people do not get the confirmation email to confirm their newsletter subscription request and about 40 people don't get the notifications that a new post has been made.

I have tried every combination in the SMTP settings and nothing has gotten the emails not be blocked and returned. Any advice would be appreciated.

The IP for the site is not blacklisted as far as I can tell.
This is what I get back in most of the returned emails;

Reporting-MTA: dns;
Action: failed
Final-Recipient: rfc822;
Status: 5.0.0
Added an autossl to the domain associated with WHM and the mail server and now it is all working. I know, a smart person would have added SSL to that domain first. :(