Forum Discussion
Custom/vanity domain name configuration in Exchange Online
Currently, when users 'Add account' in Microsoft Outlook 2016, the users are automatically configured (possibly through autodiscovery) to connect to 'https://outlook.office365.com/mapi/nspi/?MailboxI...
Such configuration is only possible if you are using on-premises Exchange server. In Exchange Online the endpoints are fixed and you cannot change them. You can easily create a solution for #3 though, by creating CNAME with the corresponding value and using it to redirect to the O365 OWA endpoint.