But yes Andrew Boon   you are 100% right, I experimented with it and that's exactly what it's produced, so it's useless.
Moreover, with your server you have no follow-up of your campaign, you are blind about what you are doing, so it is a waste of time and a method that does more harm than good.
But it can be easily resolved since we already have the module "smtp mailer" it is easy to divert all mail traffic to an external SMTP service.
So Massmailer in Una will work without problems and without inconvenience.

Maybe this may interest you, it is not expensive + -10 $ for 30.000 mails per month, and what is rare, it also accepts any type of site, you have a follow-up of the traffic, a very good rate of delivered, fast and efficient service, and they have an api.

https://serversmtp.com/