137 votesJ.Bunne commented
Hello, you have probably found your own solution for this issue by now. But for anyone else having this problem I solved it but using the following command in Powershell:
#This value for "MicrosoftOnlineServicesID" is the value of the MailBox property "WindowsLiveID" https://msdn.microsoft.com/en-us/library/ee423637(v=exchsrvcs.149).aspx
#We want to change this to our secondary domain to enable the removal of the default primary domain smtp alias
Set-Mailbox firstname.lastname@example.org -MicrosoftOnlineServicesID email@example.com
"WARNING: UserPrincipalName "firstname.lastname@example.org" should be same as WindowsLiveID "email@example.com", UserPrincipalName should remain as"firstname.lastname@example.org".
#Confirm the removal by issuing this command
Get-Recipient email@example.com | Select Name, EmailAddresses