My MTA doesn't "throw away" email. It sends back a 5xx with an appropriate message that the sending end isn't RFC compliant. Absolutely nothing wrong with that - if it's a legit sender they know I didn't get the mail.
At which point the legit sender does what -- replace their MtA? More likely they just contact thir receipient out-of-band (eg; gmail) and avoid your over-zealous mta.
Whether you think you're 'Throwing away' is just semantics. From a user's perspective that's exactly what you're doing.
It's my MTA, I'm the "user". My server, my rules. Just like if you want to come into my house you come in the front door and take your shoes off, not crash through the window in muddy boots.
Whether you think you're 'Throwing away' is just semantics. From a user's perspective that's exactly what you're doing.