Notes

I tried to mail-enable a user, but I got the following error messages:

Database is mandatory on UserMailbox
(In german: Database ist für UserMailbox verbindlich)

And:

ExchangeGuid is mandatory on UserMailbox
(In german: ExchangeGuid ist für UserMailbox verbindlich)

Using ADSI Edit, I found out that the user's msExchHomeServerName attribute contained a value pointing to a server not existing anymore. After deleting the attribute's value, I was able to mail-enable the user.