Wow, Postman is awesome -- very well thought out in every respect. Love the WP Mail settings import, and the logging is super-useful. Nice job Jason!
Definitely my go to mail plugin when needed. :-)
You know, in case this benefits someone else, our client was using Office 365, which suddenly stopped working. Postman revealed the issue as: 5.7.62 SMTP; Client does not have permissions to send on behalf of the from address.
Test emails were being sent fine - exactly what many others in these threads are experiencing. If nothing else, Postman will give you a starting point to troubleshoot.