External Mail Server - Using Outlook/Windows Mail

Hi there!

We are using an external domain & mail provider for a couple of years now and we recently signed up to MS 365 Business Standard.

I've added our domain (without DNS entries) so that all tenants can use their company email address to log in to onedrive, office and so on.

We want to keep using our existing mail server!

After logging into Microsoft Outlook... i am no longer able to receive/send mails using my company email address. Its also no longer possible to change imap/smtp settings for said email address.

How do i solve this problem?