# DE-R-028

Source: https://xmlvalid.com/rules/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.

- Level: warning
- Rule sets: Peppol BIS Billing 3.0 rules 3.0.21
- Context: `(/ubl-invoice:Invoice/cac:AccountingSupplierParty/cac:Party/cac:Contact | /ubl-creditnote:CreditNote/cac:AccountingSupplierParty/cac:Party/cac:Contact)[$supplierCountryIsDE and $customerCountryIsDE]`
- Test: `matches(normalize-space(cbc:ElectronicMail), $XR-EMAIL-REGEX)`

Validate a document against these rules: https://xmlvalid.com/validate