In my opinion, the difference between Mastodon/ActivityPub/the fediverse and email is adoption. If Mastodon was more popular:

- A handful of instances provided by large companies would probably crop up and end up hosting the majority of users

- Spammers would notice that they could reach a large number of people via Mastodon, and start spamming

- The providers of these large instances would moderate heavily to prevent their own instances being used for spam, and begin blocking / not federating with small instances

I should add that spam is probably _already_ a problem on Mastodon, but perhaps not to the extent that it is for email since the average Mastodon user is (for now!) way less likely to fall for a scam and therefore a much less valuable target.