Was resolved to '<empty>' - This error indicates that the target email address is invalid, had multiple matches in the global address book, or is hidden from the global address book. Those issue prevent programmatic access to the mailbox because they make the email address ambiguous or unresolvable.
If the source is Exchange 2007, try switching to EWS (this will not work with Exchange 2003).
- Log in to the user's mailbox using OWA.
- Compose a new email draft.
- On the TO line, type the email address of the mailbox you want to migrate.
- Click on the Check names button and carefully observe the result.
If the email address remains specified as an email address (i.e., is not resolved to a user/mailbox record in the address book), the email address is incorrect or is hidden from the global address book. Check Active Directory properties of the associated mailbox user.
If the email address matches multiple records (i.e., multiple choices are displayed), the email address has duplicate matches. Check to see if the email address could have been assigned to multiple users. Also check the personal contact folder in OWA to see if any contact has this email address (if so, delete the contact as it is causing ambiguous resolution of the email address to a mailbox user).