Resolved. Incoming test email is being delivered normally again, after delays that began Oct 12 at about 12:20 PM ET and a full outage on Oct 16 from about 8:15 AM to 12:15 PM ET.
Sending mail servers retry automatically, so most delayed messages arrived late rather than being lost. If a test from Oct 12–16 is still missing a message, please re-send it.
What we changed: the mail server now accepts mail for our test domain only, the spam has been removed, and an automatic delivery check now ...