Del. Code tit. 25, § 2607

The broker shall cause a copy of the affidavit and notice of broker’s lien to be mailed to the record owner of the commercial real estate by certified mail, return receipt requested, or by process server, to the last known address of the owner or, if that is unknown, then to the address for the owner shown on the county real estate tax records, or, if the lien is claimed on the leasehold estate of the tenant, then to the tenant at the address shown on the lease agreement.