Hi all, This is just a heads up. Microsoft's DNS server[1] handling the mail.eo.outlook.com domain was updated a few days ago to reject DNS requests coming from privileged ports. Apparently this is an effort to encourage folks on the internet to use source port randomisation for their DNS requests. If, for some reason, you are not able to use an unprivileged source port for your DNS requests, then you will not be able to send email to any person or company using Microsoft's Office 365 email system. A workaround is to hand off your DNS resolution to Google's DNS recursive resolvers at 8.8.8.8. Cheers, Lloyd [1] Yes, that's "server" in the singular. They have two NS records with two names, but only one IP address. It might be a cluster of servers using one IP address but it looks different from the rest of Microsoft's DNS infrastructure, so I wouldn't bet on it.