User mapping failed during Domino migration

Hi all, I'm trying to import some account from Domino server.
I have a problem with the internal user mapping as a see: CN=FirstNameLastName/OU=Sales/O=Group/C=it instead of real mail.

I don't know who install this Domino but there's no LDAP service active on the Domino server, so I point the migration over Active Directory.

On AD I have:

CN=lastnameF (F=first char of name)
DC=MYDOMAIN,DC=COM
OU=MyADGroup (different from Domino - Sales)

Mails are firstname.lastname@myexternaldomain.com

How can I set (if I can) a base DN on the migration tool for translating this user?

Thanks

Enrico