This left me in a scenario where running Enable-MailUser failed for the user accounts. For some of the accounts, blanking out the HomeMDB attribute was sufficient. However, for two accounts, even after blanking out HomeMDB it didn't work.
When I ran Get-User (the Exchange cmdlet) it reported back that the account was already a MailUser. However, if I ran Get-MailUser I got an error back indicating that: "The operation couldn't be performed because object 'username' couldn't be found on 'servername'".
The fix for these two user accounts was to blank out the two additional attributes:
- A list of valid values and their meanings for msExchRecipientDisplayType and msExchRecipientTypeDetails