The very short version: Microsoft are ridiculously heavy handed with their antispam, etc, mechanisms. As a result, they more frequently than not simply block mail from all but the largest mail service providers – and conversely, their own mail users simply fail to receive a *lot* of mail which they should be getting.
The problem seems to be with their implementation of “sender reputation”, which is basically the ratio of “good” emails sent from your mail domain compared to “bad” ones (spam, etc). Getting a decent insight into a domains “reputation” requires, as you might imagine, a decent volume of mail from that domain to sample from.
This can make building that reputation difficult for smaller mail providers like our own. But there several other techniques and protocols which can mitigate the issue – Things like SPF, DKIM and properly configured reverse DNS. We employ *all* of these techniques.
Yet, for the ~20 years I’ve been running our own mail services for myself, my own businesses and hosted clients, M$ have consistently and *repeatedly* simply blocked our domains and IP ranges for no readily apparent reason/s. We’ve *never* sent out spam (we have server-side mechanisms to both monitor and mitigate that).
Sooo, for the last couppla decades I’ve needed to submit about half a dozen requests per year to them to have us removed from their block lists. Of course, the only way I know such a request is needed again is when email to an M$ domain has already failed. By then, some damage is already done. And it’s historically taken at least 48 hours to get any response to the requests…
…Until this year. I got an email a few months ago from an old friend I’d not heard from in ages. From her Hotmail mail account. My reply to her bounced, so I submitted another request to M$ to have the block removed. This time, I noticed that a lot of their relevant web pages were 404’ing and I had to go to some effort to find the requisite form. Anyways, a couple of days later I got an email back telling me we weren’t blocked (that’s “normal – I think it means our sender domain is okay but the block was against our IP range (our IPs are provided from ranges belonging to the provider of our servers). But I’ve received nothing since.
So that’s it. I’m done repeatedly going through these motions for a brain-dead service provider who clearly don’t give a shit about other providers in their space, or their own users. I give up, and from now on simply do not support sending mail to any M$ domain.
A quick search today for others experiencing the same problems produced this thread in an M$ discussion forum. Don’t just take my word for it – read through the thread 🙂
In summary, if you use any of the M$ mail services – like Outlook, Hotmail, MSN, Live – grab yourself an account at any other provider – gmail is good – and start phasing out your usage of the M$ one/s sooner rather than later.