Add my Domain to 365 without email

Occasional Visitor

HI - this my first post & hope someone can help - or reassure :)


I have 10 users with "Microsoft 365 Business Standard" - we all use Windows 10 and office 365 suite (not MS Teams - but would like to).


All users have software installed with the default * (domain)


Our Email is hosted elsewhere (i understand it can be migrated - but were not ready yet).


Can i add our Domain (lets call it *myFastHostsDomain) and switch all the * accounts to *myFastHostsDomain.


My worry is Outlook / email / exchange may not work - or get confused.


Will emails start going to another mailbox on the 365 acccount once this is done?  And could i use Teams etc with this setup?


Any help would be much welcomed


6 Replies



Hi, you can certainly add your custom domain into Office 365 without affecting your other email system yes.


As long as you don't change your MX records or Autodiscover records you will be fine.  Just add the domain in,  verify with a TXT record and you will be able to set your users addresses to as the primary UPN suffix as opposed to  


Just be careful if you are assigning licences to the users that include an Exchange Online licence as this will create a cloud mailbox.  This in itself will not affect your existing email routing but if you ever come to the point later down the line where you want to migrate your mail into Exchange Online, then having duplicate mailboxes will cause you extra hassles at that point.  You can exclude Exchange Online from the licence assignment to avoid this.  This can be done manually, or by group based licencing.

@fedcap @PeterRising 


Hi, we have a similar problem, but we use Microsoft Teams and we need scheduled meetings. This means we need to assign Exchange Online plans for allowing access to calendar, and this gives us some serious problems, since some emails are routed directly to the cloud mailbox instead of honoring what the MX records says.

Is there a way out of this, like forcing Microsoft to always honor MX records when sending email ?



@GianlucaAmatoDid you find a solution?

We have similar situation -- we have a custom domain linked to Office365 but MX records are set to our custom email provider (we do not use Office 365/Exchange for emails).

However Teams meeting invitations (not just Teams, any invitation from Outlook calendar) are sent to Exchange/Outlook accounts (which are not used) instead of our real email accounts respecting MX records.


@petrdvorakI made some progress, but I am not satisfied of the results so far.


I tried adding a flow rule in Exchange which forwards all incoming e-mail trough our on-premise SMTP server. This works, but as a side effect, when a meeting is scheduled, the calendar of involved participants is not updated. My guess is that the calendar in Microsoft 365 is updated when an email with the appointment is received, but with the flow rule everything is diverted to our email server.


I would like to try with a flow rule that send everything to our on-premise SMTP server but also keeps the original message in the Exchange mailbox, but I do not know if it is possible.


@GianlucaAmato could you write how to create this flow rule? I have simillar problem, we use only external provider for mail boxes, some of the users enable cloudbox and now when someone send invite from teams this mail goes to cloud box not to local mail boxe


From the old Exchange admin center I have created a Connector (mail flow -> connector) with the following configuration:

- Retain internal Exchange email headers

- Only when I have a transport rule set up that redirects messages to this connector

- Add the hostname of my on-premise SMTP server as smart-host


Then, I have added a Mail Flow Rule (mail flow -> rules) with the following configuration:

- Apply rule if: the recipient domain is...... domain I want to redirect on-premise....

- Do the following: Use the following connector ....  the connector I have created before

- Choose a mode for this rule: Enforce