> Mail server admin can do a SPF check (or have a list of allowed source 
> email domains) before outbound and reject forwarding these emails.

You can check this before you allow the domain to be sent. However, I don't 
know of any mail server/MTA that can do this out-of-the-box. And note also:

1. You would have to do this before sending every single email, because DNS 
records can change. As a mail server provider you have no control over it.
2. Even then it can take a while until you as a sender server notice this, 
thanks to TTL and DNS propagation.
3. The way this (to us unknown provider) works, you can't technically handle 
the problem (siehe 1. und 2.)


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