DE-R-028
If both supplier and customer are located in Germany, "Seller contact email address" (BT-43) should contain exactly one @-sign, which should not be framed by a whitespace or a dot but by at least two characters on each side. A dot should not be the first or last character.
What the rule checks
As the rule set's publisher wrote it. The rule runs on every element the context matches, and fails where the test is false; the validator reports the line of that element.
(/ubl-invoice:Invoice/cac:AccountingSupplierParty/cac:Party/cac:Contact | /ubl-creditnote:CreditNote/cac:AccountingSupplierParty/cac:Party/cac:Contact)[$supplierCountryIsDE and $customerCountryIsDE]matches(normalize-space(cbc:ElectronicMail), $XR-EMAIL-REGEX)Checked with Peppol BIS Billing 3.0 Invoice, Peppol BIS Billing 3.0 Credit Note