Mailboxes are not getting scanned.
When looking at the users' logins in the Administration Console the email address is missing.
The mail attribute is set correctly in Active Directory.
There is only one domain and linked mailboxes are not in use.
Incorrectly configured Active Directory Connector.
If using a single domain, the directory Connector needs to be set as such, if it is set to Active Directory User / Resource Forest and linked mailboxes are not in use the email addresses will not be updated correctly.
To check the current setting, perform the following: