You never really know the cause for things like that, otherwise you're just handing out instructions to spammers on how to circumvent restrictions. You can only double check the basics, idly guess about everything else, and try a new approach while crossing your fingers.
Yeah I don't think TLD matching is included in any bounceback or header that I have seen, for either Exchange or SpamAssassin, unless you have a named mail flow rule for it.
