I use Gmail as a free spam harvester to train my own spam filter - https://news.ycombinator.com/item?id=38843288.
But as others here have suggested greylisting is extremely helpful in this space as legitimate servers should always retry. Well only my power company is the exception and they will fall back to sending paper bills, but even Gmail falls foul for them. It's also one big reason I'm not worried about up to a week downtime. But I have two email servers, a receiving and a storage server, the receiving is cattle and I car re-deploy in minutes if needed. - https://news.ycombinator.com/item?id=38512732
On greylisting I would say using https://github.com/stevejenkins/postwhite (even if it's very old and not actively maintained) has proven very important for the annoying 2FA emails, I strongly contend that email isn't suitable for this use case but that's another conversation)