This question has been answered by omegerard. See the first response.
Are the test recipients on the same domain as the web site? If they are, try sending a test to email addresses on a different domain.
You could try running Bobs Quick Email - https://modx.com/extras/package/quickemail to test for problems.
How many emails are you sending out in one go? Is it possible that your bulk emails are being interpreted as SPAM and blocked by service providers?
You could run a check on MX Toolbox to see if you have been blacklisted. https://mxtoolbox.com/