I was being blocked by the Microsoft email servers, and their help pages were so convoluted that I had trouble finding out what the real problem was.
Since no other servers blocked us, and since none of the blacklist servers blocked us, I couldn’t tell what was happening. Then, after doing more reading, I thought it might be Return Path Inc. (RPI). RPI was bought by MS a long time ago, to be their pay-to-send for-profit whitelist service. (They deny it’s a white list, but come on. It’s a whitelist.) They have some tools on their website that analyze domains.
I couldn’t really spot the problem, but eventually made a guess. I think Microsoft, Yahoo, and Apple are being a lot more “uptight” about repetitious admin emails. If an admin doesn’t take care of their mail server, the admin emails get flagged as a problem, and the site may be blocked.
So, I dealt with the admin emails and waited. It didn’t fix the problem.
Domain Email Configuration
Then, I thought that the spam situation may have flagged the domain, and then domain needs to be fixed up to be released from sender jail. The sending domain was called both slaptech.net and mail.slaptech.net, but mainly referred to by IP address. The IP address’ reverse record resolves to mail.slaptech.net. The mail.slaptech.net domain did not have SPF and MX records.
I added SPF and MX records. I’ll need to wait to see if this issue is resolved over time, as the new records get updated, and we’re unblocked.
Right now, the sending IP address has a Return Path score of 99/100, so it’s fine.