We recently found out that since October 2014, open rates for Microsoft domains (outlook.com, hotmail.com, etc.) dropped considerably compared to the other domains (gmail, etc.). We're talking about 1/3 of the overall open rates.

Did anyone experience the same?

We typically target 2 audience types (members, contacts) and 2 languages (Dutch, French). The problem occurs only with the Members database, and is not linked to the language. E-mails to the contacts have a "normal" open rate compared to the average. Members are all 100% legit (ID card verification required), have higher open and click rates, yet open rates for Microsoft domains are 1/3 of the average.

A/B testing allowed us to eliminate 99% of the e-mail. The only thing left is the unsubscribe link which is different for members and contacts. We sent the exact same mail (aside from the unsubscribe link) to a selection of members and contacts, and still the same result: mails to the members seem to have a problem getting through to Microsoft mailboxes, the same mail to contacts doesn't have that issue.

We recently changed ESP and are now on fresh dedicated IPs, slightly different sender address etc., and still exactly the same results (in fact, it's by using the new ESP that we found out about the problem).

Would anyone have any suggestions? Potential cause? Anything to try out?

All help much appreciated!