unfortunately it is not like email. they don't block for spam, they block for things they don't like. i don't know if it is a cultural problem, or an consequence of the fact that mastodon started in the mid-2010s vs email which is ancient
That's their mastodon instances, not anyone else's, it's up to them to do what they want
If Mastodon ever takes off for real, I expect that professionally run servers will be pretty popular.