When you try to open a new mailbox for an Office 365 users you get the following error;
"This user's on-premises mailbox hasn't been migrated to Exchange Online. The Exchange Online mailbox will be available after migration is completed."
AD Connect is configured to sync users, groups and passwords from the existing Active Directory (SBS 2011), however the option for “Exchange Hybrid Deployment” was not selected on purpose. This is because in this particular case the migration was for 6 users, therefore a PST export/import was done to migrate the e-mails, contacts and calendars.
The root of the problem is because the Active Directory attribute msExchangeMailboxGuid is being synced to Azure AD in Office 365, when it’s not required.
You have to edit a configuration inside AD Connect (it’s actually FIM 2010 R2 under the covers). To open the configuration panel for FIM, browse to C:\Program Files\Microsoft Azure AD Sync\UIShell and launch miisclient.exe.
Click Connectors and select the connector for your local Active Directory and choose Properties.
Click Select Attributes and scroll until you find msExchangeMailboxGuid, if you have the same problem as me this will be selected. Simply disable this attribute.
You then have to delete the old reference to the msExchangeMailboxGuid from the FIM Connector Space. To do this select the Active Directory management agent (also known as a “connector”), and choose Delete. Read the next part properly.
Ensure that Delete Connection Space Only is selected and click OK.
It will ask you to confirm you want to delete data from the connection space, click Yes. If you did delete the entire connector, you could provision again by running the AD Connector wizard. This is fine if you have not made any major modifications to your AD Connect configuration.
Use the following PowerShell command to force an entire sync across AD Connect;
Start-ADSyncSyncCycle -PolicyType Initial
You should notice updates when the Operations complete.
Now if you return to Office 365, you should see the following.