If literally anyone could find your Mastodon address and message you, you'd get spammed daily there too, and it'd be the same problem.
So you think, fine, whitelists! But you still need to be able to accept messages by new authors without knowing their From: address ahead of time. You'd have to comb through your spam folder past tens of thousands of messages from new authors to find the one new genuine sender. Rings of trust don't solve it either because either you get spammed by someone in a ring of trust, or messages end up in concentric rings of spam folders.
You can host your own mail. It's just very hard to do it correctly, easy to screw up, and there's basically no gain whatsoever by doing it yourself. Some problems are just difficult and cannot be easily solved by a single person. You can't be your own CA [and have anyone trust your connections]. You can't create your own TLD [and have everyone be able to resolve it]. You can't create your own ASN. You can't create your own IP address. There are some things in life you have little to no control over, even on the Internet.
So you think, fine, whitelists! But you still need to be able to accept messages by new authors without knowing their From: address ahead of time. You'd have to comb through your spam folder past tens of thousands of messages from new authors to find the one new genuine sender. Rings of trust don't solve it either because either you get spammed by someone in a ring of trust, or messages end up in concentric rings of spam folders.
You can host your own mail. It's just very hard to do it correctly, easy to screw up, and there's basically no gain whatsoever by doing it yourself. Some problems are just difficult and cannot be easily solved by a single person. You can't be your own CA [and have anyone trust your connections]. You can't create your own TLD [and have everyone be able to resolve it]. You can't create your own ASN. You can't create your own IP address. There are some things in life you have little to no control over, even on the Internet.