On 13 Dec 2015, at 13:56, Mark Sapiro wrote:
In fact, the fact that almost all the unsubscribed users were hotmail makes it seem that this is not DMARC, but more likely hotmail (Microsoft) blocking the sending IP.
And with the one other being live.com.au (roughly: Australian Hotmail) it becomes a distinctively Microsoft problem.
For what it is worth, recently the policy staff and policies of Microsoft's former-Hotmail and former-Frontbridge have seemed to be converging/merging. One win for the world of this is that the chronic problem of Hotmail/Live.com mail being accepted but silently discarded has been replaced by overt rejection at the MX border (no, not THAT MX border) in many cases. For Mailman lists, one result is that long-dead addresses are getting cleaned out because they are now bouncing. Another result is that some silent discards have been replaced by delivery, so in some cases people who haven't seen mail from a list in years and now are seeing it and telling MS that it's spam. The second process can lead to broad blocking across MS-hosted domains, but at least now it is visible.