SMTP Add-on issues?

Great plugin overall!
I am trying to improve the delivery of the Newsletter plugin, but I am hitting a wall.

For context, my client has been using the default WP mailer with this plugin for a while.
Unfortunately, several mail services are now rejecting emails sent in that way as they do not look super legit for spam filters. Long story short, many emails are no longer reaching destination.

My initial instinct was simply install something like WP Mail SMTP on the site, but for some reason that is not compatible with your plugin for some reason. (Email Source: Newsletter Mailer: Other SMTP SMTP Error: data not accepted. SMTP server error: DATA END command failed Detail: Non-Compliant sender address. Code : -2 SMTP code: 550 Additional SMTP info: 5.7.1)

So I turned to your proprietary SMTP add-on, but still no joy. No matter what combination of protocol (SMTP+STARTTLS or SMTPS) and port I try with the add-on it always fails (SMTP Error: data not accepted. SMTP server error: DATA END command failed Detail: Non-Compliant sender address. Code : -2 SMTP code: 550 Additional SMTP info: 5.7.1)

For the record, when entering the exact same combination of port, authentication details and protocol with WP Mail SMTP, the emails are sent without any problems.

Any suggestion other than switching to an email sending service (MailGun, etc..)?

By the way, I am surprised that there is no SSL option in this add-on. Just SMTPS?

The page I need help with: [log in to see the link]