If the smtp server is allowing spam to be sent through it, then it is not trustworthy. It doesn't matter which domain it was sending on behalf of. Though the

If the smtp server is allowing spam to be sent through it, then it is not trustworthy. It doesn't matter which domain it was sending on behalf of. Though the real reason is that we've only had spf records and what not for a relatively short amount of time, so most systems are designed from the perspective that you can't trust the domain a message is claiming to be from.