If you are using office365 in hybrid mode then you should really have an on premise exchange server. You will soon find out that there are account related things that you would usually handle from exchange that you cannot do from O365 exchange panel. You get this
This is basically because AzureAD is a read only copy of your active directory and it cannot make changes to it. There is apparantly someway to make this work with Microsoft Online services assistant, and maybe one do I will look into how that works but today we are going to use ADSIedit.
- On a domain controller run adsiedit.msc
- Find the user right click and go to properties
- To change the default email address for a user you need to change the variable proxyAddress
- The default address needs to look like this : SMTP:firstname.lastname@example.org
- secondary address look like this : smtp:email@example.com not the case change.
- Apply and save it. Now you have to wait for replication to happen, or simply force a replication.