If you have the reverse DNS set to something within a domain you own, they'll usually let you do it yourself (the process involves them sending an email to post

If you have the reverse DNS set to something within a domain you own, they'll usually let you do it yourself (the process involves them sending an email to postmaster or abuse at the parent domain of the server or at the AS which controls the IP address, but you get to choose which one). Still doesn't seem to stop them randomly flagging emails as spam, sometimes mid email chain. The MS support team are also responsive in my experience.