Recently we deleted our first default mailbox database and migrated to another database.
Afterwards we had issues when creating new mailboxes.
In the New Mailbox wizard you can set the mailbox database you want the mailbox to go to, if you do now choose a mailbox database, the wizard wil automatically fallback to the default mailbox database.
In our case, it did not because the wizard could not find a valid database anymore.
When doing a get-mailboxdatabase we noticed that the "IsExcludedFromProvisioning" parameter was set to True.
For easy viewing, you can do a :
Get-MailboxDatabase | fl isexcludedfromprovisioning, issuspendedfromprovisioning
With this command you only view the relevant parameters for this issue.
As said before, in my case IsExcludedFromProvisioning was set to true and therefore New-Mailbox requests where i did not specify a mailbox database failed.
To resolve this, the only thing you have to do is set this parameter to false.
Do this with the Set-MailboxDatabase "database" -IsExcludedFromProvisioning $false command.
To verify the parameter has changed, redo the get command to show the 2 parameters.
Now you are all set!
New-Mailbox requests will now complete succesfully, even if you don't specify a mailbox database in the wizard.