Sending e-mails with non-registered mail addresses - Sender address rejected not owned by user

Error Message: “Sender address rejected: not owned by user”

If you send a mail with a sender address different from the ones configured for your account or aliases, you will receive a non-delivery notification with the following message:

Sender address rejected: not owned by user

To prevent address spoofing and abuse, you can only send mails using sender addresses that are registered for your account as either the main address or an alias. All settings | Mail | Mail aliases

If you try to use sender addresses that cannot be associated with your account or that are outside of mailbox, the delivery will be rejected with the following error message:

553 5.7.1 wrongaddress@example.com
: Sender address rejected: not owned by user realuser@example.com
;

You should always use only sender addresses that you have configured as your main address or as an alias. This ensures that replies or delivery notifications (bounce mails) are received correctly.

Note for Organizations and Companies

In a business environment, it can make sense to use many different sender addresses — for example, to assign processes to specific user accounts or because automated booking systems send numerous mails. In this case, these addresses must also be registered individually as aliases.

However, if a catchall for the domain is configured (e.g. @example.com), any local parts (usernames) of that domain can be used. This ensures that replies sent to these addresses can also be delivered successfully.