Mapping domain when migrate to ExO on O365

Occasional Contributor

Hi all friends, and experts.,


We are facing an case about migrate mail to ExO on O365 as below:


Domain name for old system email :

When my customer move to ExO, O365 they want to use: for email domain.


This is 1st , I work with O365, Please help me to do that and give me some advise?. Thank you so much.

Best and regards,


4 Replies

For more information:


My customer they have a old system mail  ~ 50 users., domain for this system is :


Now they want to 1/2 of 50 users to O365 and keep 1/2 in old system. and keep this domain (


It mean : 25 users on-premise are going to use old system mail with domain :

and 25 user in O365 are going to use email domain is


Thank you so much !




You have to set up and verify that domain in office 365 to start with! See here:

Don’t change the dns records for exchange! The mails will still go to your old mail server!

What you will do next all depends on what you want to accomplish! Migrating? Set up hybrid solution? What mail system are you running today? We might help you further with more info!

Basically in the end you want to switch the dns records to point to exchange online! Also the UI in office 365 gives great help here regarding records and such!
Just make sure you are comfortable doing this, otherwise get professional help! You don’t want your users screaming that they don’t get any mails :)

Ok! What system are your customer running?

Dear admin,


I am asking him, but as my understand, we have two type:

the first one  is Exchange server

the second one is Open server


So what are different between, when customer say: they are running Exchange or Open mail ?


Thank you so much.