I’ve not received any email notifications since Sunday.
But the Full Headers of Sunday’s email contain a definite clue about why many mail servers are rejecting NE messages:
Authentication-Results: incoming_mbo;
dkim=none;
dmarc=fail reason=“SPF not aligned (relaxed), No valid DKIM”
header.from=nakedexperiment.com (policy=none);
spf=pass (incoming_mbo: domain of u767186973@srv582.main-hosting.eu designates 23.83.218.246 as permitted sender)
smtp.mailfrom=u767186973@srv582.main-hosting.eu;
arc=pass (“mailchannels.net:s=arc-2022:i=1”)
Although my mail server did not reject that particular message, many mail hosts now consider dkim and dmarc to be compulsory. Even those that aren’t so strict will nevertheless be more likely to reject a message based on other criteria if dkim and dmarc are not present and correct.
Hostinger has a page explaining how to configure those settings, at https://www.hostinger.com/support/email/hostinger-email-dns-records/