New Test today:
Posted a new message to the forum, which was received in 4 out of the 5 accounts; the 5th account should have received a no-reply email, since its domain is not on the approved domain list.
Replies via the sustainlv.org email server all were received at the mail server, in a folder named for the +data, but not collected by Moodle. The 5th account was a gmail account, and the user's reply was received and was posted to the forum. Trouble-shooting with email provider.