I am currently using Office 365 Small Business plan P1 with a custom domain that is managed by Microsoft/Office365 and registered with GoDaddy.

I am planning to move away from Office 365, but I want to move my mailbox before I re-point DNS entries to start receiving email at the new location. So I was planning to set up a new Outlook.com account to which I could potentially copy my emails over. Once the copy operation is complete (could take a while because I have a fairly large mailbox of 5GB), I was hoping to add a new email address with my custom domain.

Is it possible to do so, and more importantly, to make the custom domain address the default and the Outlook.com account to be a secondary? In fact, if I can make the custom domain account the only account and drop the Outlook.com account completely, that would be fine as well.

Thanks in advance!