Since upgrading from 12.6 to 15.x I have not successfully sent newsletters.
This is a shared webhost using Softaculous to install and upgrade.
Sending is throttled to 180/hour (6/120secs) and the output shows all the emails being sent.
The only newsletters being received are only those sent within the same domain as the Tiki installation.
The webhost has checked the hosting and not found any problems in their hosting installation.
I tried a second installation of 15.2 but that did not work either.
I have installed a third installation on the same webhost using Tikiwiki 12.9 and successfully tested sending newsletters. This is now my workaround option, but I hope a fix can be found before the start of next month when I send the next distribution.
I suppose it may be possible to try this out on the Tikiwiki provided testing installations using my test email addresses, but think it is now probably not necessary.