Hallo Christian,
wie gesagt, eine endgültige Lösung habe ich noch nicht gefunden.
Hier mal der Auszug aus der Domino Hilfe um die Inbound Return Receipts von externen Mails abzuschalten.
To enable/disable return receipts
1. Make sure you already have a Configuration Settings document for the server(s) to be configured.
2. From the Domino Administrator, click the Configuration tab and expand the Messaging section.
3. Click Configurations.
4. Select the Configuration Settings document for the mail server or servers you want to administer, and click Edit Configuration.
5. Click the MIME - Conversion Options - General tab.
6. Complete these fields, and then Click Save & Close:
Field Description
Return Receipts Choose one:
Enabled to allow the sender of a message to receive a return receipt.
Disabled to prevent the sender of a message from receiving a return receipt.
Return Receipt Mapping Choose one:
Use Disposition-Notification-To - (default) When converting an outbound Notes message that includes a return receipt request into MIME format, the server converts the Notes ReturnReceipt item into the MIME header item Disposition-Notification-To.
Use Return-Receipt-To - When converting an outbound Notes message that includes a return receipt request into MIME format, the server converts the Notes ReturnReceipt item into the MIME header item Return-Receipt-To.
Note This field appears only if you enable Return Receipts.
Note Domino does not map the Return Receipt request to one of the MIME headers if the address specified in the Disposition-Notification-To or Return-Receipt-To header does not match the sender's address. Domino sends return receipts only to the sender.