Thanks! I think.
Maybe another suggestion ... Moodle does have a testing script (hidden) for testing EMail configurtion but it's a little hard to understand the output. One has to turn it on, but also remember, where they went to turn it on so it can be turned off once the issue is resolved. This so cron jobs won't get bogged down (better then nothing, however).
Seems to me this problem is re-occuring ... happens all the time ... and happens with any web based app that has a module/add-on etc. for EMail.
Documentation and FAQ's help, but ... Moodle code could have some scripts to do two things ... 1. check for MX records for the domain the moodle server runs under. 2. check DNS for 'any' text records to see if the MX servers for that domain have what might be required by receiving mail servers ... an SPF record which would include the moodle server, a DKIM check, and (added recently) a DMARC check.
**If not coding** (moodle is getting bloated) a link to tools like http://dnscheck.pingdom.com/ or a page that has such tools for Moodle server admins to use in investigating their own EMail issues. Maybe those pages are there ... haven't looked.
'spirit of thinkering', Ken