DE-R-028: If both supplier and customer are located in Germany, "Seller contact email address"…

The rule is an advisory warning (severity warning), not a rejection, checked only on UBL invoices and credit notes where the accounting supplier party country and accounting customer party country are each DE. It applies to cbc:ElectronicMail in cac:AccountingSupplierParty/cac:Party/cac:Contact (the seller contact email address, BT-43). The value is normalized by normalize-space() and must match $XR-EMAIL-REGEX = ^[^@\s]+@([^@.\s]+\.)+[^@.\s]+$. That regex requires exactly one @, no whitespace, a non-empty local part before @, and a domain with at least one dot between non-empty labels that contain no @, ., or whitespace. The message additionally describes constraints such as at least two characters on each side of @ and no dot framing @, but those are not all enforced by the regex test; the regex is what the validator actually checks.

Set cbc:ElectronicMail to a value that matches the regex, e.g. name@example.com. Ensure the value uses exactly one @, contains no whitespace, and the part after @ has at least one dot and no empty labels (so the value cannot start or end with a dot after @). If the current value looks like an email but triggers this warning, check for hidden whitespace inside it, multiple @, or a domain without a dot. Because this is a warning, the invoice is not rejected.