# BR-DE-28 Seller email should be shaped like an email address

XRechnung warns when seller email (BT-43) does not contain exactly one @ sign with valid surrounding characters.

## Problem Description

XRechnung warns when seller email (BT-43) does not contain exactly one @ sign with valid surrounding characters.

> **WARNING**: XRechnung 3.0

## Context and Significance

A syntactically plausible contact email reduces manual clarification loops with public recipients.

## Solution

Use a reachable address such as invoice@example.com and remove spaces, trailing dots, or placeholders.

- validate
- convert

## Valid Examples

- BT-43 = invoice@example.com

## Invalid Examples

- BT-43 = invoice@
- BT-43 = invoice.example.com

## Code Examples

### Valid XML
```
<cac:Contact>
  <cbc:Name>Sales Department</cbc:Name>
  <cbc:Telephone>+49 30 123456</cbc:Telephone>
  <cbc:ElectronicMail>invoice@example.com</cbc:ElectronicMail>
</cac:Contact>
```

### Invalid XML
```
<cac:Contact>
  <cbc:Name>Sales Department</cbc:Name>
  <cbc:Telephone>+49 30 123456</cbc:Telephone>
  <!-- nothing follows the @ sign in BT-43 -->
  <cbc:ElectronicMail>invoice@</cbc:ElectronicMail>
</cac:Contact>
```

## Affected Fields

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

## 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.