IONOS blocks non-delivery reports and out-of-office messages

Since January 2024, IONOS (1&1) rejects emails where the sender address field (MAIL FROM:) is empty. NDRs and Out-of-Office messages use an empty sender field as required by the SMTP standard — but IONOS blocks them.

Solution

Use MultiSendcon (the free Lite version is sufficient) as an outgoing relay between Exchange and IONOS. MultiSendcon can automatically fill in a sender address from the message content or a configured default address, making the messages acceptable to IONOS.

This is a workaround for a non-standard restriction imposed by IONOS. The behaviour is not configurable in Exchange or POPcon.