I know how frustrating it is to to get excited about receiving an offer or inquiry, only to find out it's bogus.
For us at Efty, the priority is giving our sellers access to all inquiries at any time, rather than unintentionally blocking a legit inquiry that otherwise could have resulted in a sale. For this reason, we even have a "spam" folder just like Gmail offers its users. Here you can see each and every one of the inquiries that were "blocked" by our spam-fighting software. Just like with Gmail, you might find a legit inquiry from there from time to time because the sender's IP or email was previously used for spam, but the inquiry for your domain name was legit.
We always looking for ways in which we can improve this set-up so please never hesitate to email us with examples.