We have been using IPv6 for years, but hotmail has been IPv4. Checking our email logs, we have been successfully delivering email via IPv4 to Hotmail without issue since May 29, 2022 (as far back as our email logs go at the moment). At Oct 24 21:06:13 UTC, the MX record started returning IPv6 addresses: hotmail.com. 3256 IN MX 2 hotmail-com.olc.protection.outlook.com. hotmail-com.olc.protection.outlook.com has address 104.47.1.33 hotmail-com.olc.protection.outlook.com has address 104.47.2.33 hotmail-com.olc.protection.outlook.com has IPv6 address 2a01:111:f400:7e8a::33 hotmail-com.olc.protection.outlook.com has IPv6 address 2a01:111:f400:7e89::33 So that's cool. Except that 54 minutes later, Hotmail blocked ALL of our email traffic. Deferred: 451 4.7.500 Server busy. Please try again later from [2a01:111:e400:7e0d::51]. (AS750) [HE1EUR04FT020.eop-eur04.prod.protection.outlook.com] Pretty sure that the 9 (nine) emails we went via IPv6 at 5pm EDT did not all get reported as SPAM. To aggravate the situation further, Hotmail's Sender Tools are all still IPv4-only. I cannot check or add or monitor an IPv6 address or range, and their Support Tool also only allows IPv4 addresses. I'm not an enterprise customer of Microsoft, but this is pretty crappy. I'm now looking for ways to connect to Microsoft via IPv4 only while still using IPv6 for other email systems. Anyone else run into issues like this? Beckman --------------------------------------------------------------------------- Peter Beckman Internet Guy beckman@angryox.com https://www.angryox.com/ ---------------------------------------------------------------------------