1. Using Default Setting for email as follows: DEFAULT + Set return path with -f parameter
(my Admin panel doesn't identify this as PHP, but SMTP is definitely NOT selected). This has been unchanged since our launch at end of 2015.
2. If ACP is Admin Control Panel (forgive my ignorance), yes I am seeing dozens of errors listed as: "Lost Connection to MySQL server at 'reading initial communication packet', system error 4 [alto system error 0]
3. One Yahoo email address, one ATT.mail address, one Gmail email address, a half dozen others all small domains. Not Hotmail, No Windows, no Outlook.
Are those MySQL error log entries current?
I would expect more problems than failed emails if they are.
If there are no specific error log entries related to email then you would need to check the server mail log to determine whether they are being sent or not.
Your host should be able to help with that if you are unsure how to check the log.
I have checked with my serverAdmin. We seem to have located a choke point in our email relay service. We will investigate that. It appears that the issue may not be Xenforo (at least that's our best bet currently). Thanks for your help Brogan.