# BR-DE-28 Seller contact email must have valid syntax and a single at sign

Under rule BR-DE-28 of the German XRechnung profile, the seller contact email address (BT-43) must be a syntactically valid electronic mail address conforming to standard RFC 5322 specifications. The string must contain exactly one "@" character separating the local user/mailbox name from a valid fully qualified domain name (e.g. "rechnung@lieferant.de"). It must not contain leading or trailing whitespace, illegal characters, multiple comma-separated addresses, or missing top-level domain extensions.

## Problem Description

Under rule BR-DE-28 of the German XRechnung profile, the seller contact email address (BT-43) must be a syntactically valid electronic mail address conforming to standard RFC 5322 specifications. The string must contain exactly one "@" character separating the local user/mailbox name from a valid fully qualified domain name (e.g. "rechnung@lieferant.de"). It must not contain leading or trailing whitespace, illegal characters, multiple comma-separated addresses, or missing top-level domain extensions.

> **ERROR**: XRechnung 3.0

## Context and Significance

Automated receipt platforms use BT-43 to return validation failure reports (Prüfprotokolle). An unparseable email syntax immediately triggers Schematron error BR-DE-28.

## Solution

In your invoicing or master data settings, verify that the contact email address is formatted correctly as a single valid address (e.g. "buchhaltung@unternehmen.de"). In UBL 2.1 syntax, populate <cbc:ElectronicMail> within <cac:AccountingSupplierParty><cac:Party><cac:Contact>. In UN/CEFACT CII syntax, populate <ram:URIID> within <ram:EmailURIUniversalCommunication> under <ram:SellerTradeParty><ram:DefinedTradeContact>. If multiple recipients need notification, configure email distribution lists or mail forwarding on your mail server instead of putting multiple addresses in BT-43.

- validate
- convert

## Valid Examples

- BT-43 = "rechnungswesen@lieferant.de"
- BT-43 = "billing.team@company.eu"

## Invalid Examples

- BT-43 = "rechnung@lieferant@de" (Malformed address containing multiple "@" characters)
- BT-43 = "rechnung@lieferant, vertrieb@lieferant.de" (Multiple addresses separated by comma)
- BT-43 = "rechnung.lieferant.de" (Address missing the "@" delimiter entirely)

## Code Examples

### Valid XML
```
<cac:Contact>
  <cbc:Name>Buchhaltung</cbc:Name>
  <cbc:Telephone>+49 30 1234567</cbc:Telephone>
  <cbc:ElectronicMail>buchhaltung@unternehmen.de</cbc:ElectronicMail>
</cac:Contact>
```

### Invalid XML
```
<cac:Contact>
  <cbc:Name>Buchhaltung</cbc:Name>
  <cbc:Telephone>+49 30 1234567</cbc:Telephone>
  <!-- Multiple addresses separated by comma violate the single-address syntax rule -->
  <cbc:ElectronicMail>rechnung@firma.de, vertrieb@firma.de</cbc:ElectronicMail>
</cac:Contact>
```

## Affected Fields

- [BT-43 Seller contact email address](/resources/xrechnung/bt-43-seller-contact-email-address)
- [BG-6 Seller contact](/resources/xrechnung)

## Related Rules

- [BR-DE-7](/resources/kosit-validator-errors/br-de-7-seller-contact-email-required)

## Validation Tools

- [Official KoSIT Validator](https://erechnungsvalidator.service-bw.de/): Validate your XRechnung files against all BR-DE rules.
- [Convert PDF to XRechnung](/pdf-to-xrechnung): Automatic validation error fixes when converting.

## Related Resources

- [Official Specification](https://xeinkauf.de/xrechnung/)
- [Test Suite](https://github.com/itplr-kosit/xrechnung-testsuite)
- [Complete Error List](/resources/kosit-validator-errors): Overview of all German XRechnung validation rules.