Messages to hotmail.com addresses started bouncing. A user on my server e-mailed from his outlook.com address to say he was having problems and I found I couldn’t reply to this either.
Microsoft use an extremely blunt instrument of blacklisting IP addresses and entire subnets. This makes some sense. If a server is routinely blasting crap at your service, simply blocking the IP early in the process avoids having to process the junk with complex filters. The downside is that if your server gets blacklisted you may not be able to do anything about it.
Having set all this up, and confident I was complying with the Outlook.com policies I requested mitigation for my server. No joy. My server “does not qualify for mitigation” although they won’t tell me why.
My server handles a few personal domains, one for a local charity and sends very little e-mail. I suspect the problem was caused by a user sending to a distribution list of around 60 addresses and one of those recipients clicked the junk button.
The trouble is I’m guessing. I don’t know why Microsoft blocked my server, they won’t tell me, and the tools they provide to help manage this don’t work if you send less than 100 e-mails a day.
Short of relaying mail through an expensive “deliverability” service, about the only thing I can do is change my IP. This involves changing enough DNS records that I really don’t want to make a habit of it.
So no conclusions or solutions to this problem. If you’ve encountered it, know you’re not alone.