"Your domain cannot be added to Microsoft 365 at this time"
A brand new domain refused with a support code and no explanation. The cause was a record left behind by somebody else.
The message is unhelpful by design: your domain cannot be added at this time, contact support with a code. No reason, nothing to act on.
Check for a stale verification record first
Look for a TXT record at the apex shaped like this:
v=verifydomain MS=1234567That record only exists because the domain has been through a Microsoft ownership verification before. On a domain you have just registered, that means a previous owner, or a self service tenant somebody created by signing up for a Microsoft product with an address at the domain.
Deleting it is free and safe. Adding the domain then frequently works, and you will be issued a different MS value as a fresh challenge. If the new value differs from the old one, you know you are dealing with a genuinely new registration rather than the old claim.
If it still fails
Then the domain really is verified in another tenant, and a domain can only be verified in one at a time. Nothing you can do from your own side releases it. That is a support call, referencing the code in the message.
This is also worth knowing before you buy a used domain. A domain that has been through somebody else's Microsoft tenant carries baggage that is invisible at the registrar and only appears when you try to use it.
Do not block on it
The common reason people need this urgently is one contact address on a website. That does not need a mailbox or a tenant. Registrar level email forwarding takes two minutes, needs no verification, and can be replaced later.
If your site is already publishing an address, get forwarding up first and sort the tenant out at your leisure. An address on a live page that bounces is a worse problem than not having a mailbox.
A domain with no MX records cannot receive mail at all, regardless of what is configured elsewhere. Before debugging anything complicated, confirm the domain actually has an MX record.
Need help with any of this?
These notes are free and always will be. If you would rather someone just set it up, or you are stuck on something similar, get in touch at hello@opsira.io.