I have been ranting and raving about not being able to configure my Office 365 hosted domain to be configured to let me pull down mail “as it arrives”. It turns out that there is a three account limit. After three accounts are configured to get mail as it arrives in the Windows 8 Mail App, that choice is no longer offered when additional mail accounts are configured.
Of course, you need to configure a Microsoft account to even get started and that gets the first slot (hotmail, outlook.com account, etc.) In my case, I configured my Microsoft accounts first, and then a Gmail account. When I got to configuring my vanity domains, I only could get PULL settings, of which every 15 minutes was the shortest interval.
Realizing that only three accounts could be configured, I had an aHa moment and changed the settings on my hotmail and outlook.com accounts which are far less important to me, and then was able to configure my other accounts for PUSH (as it arrives).
I have made contact with the Mail team at Microsoft and am working towards getting this limit removed (and some other things).