Thanks for being obsessed with this. I'm worried about spam. Email made the web what it is. A decentralized open system to exchange ideas and information. However, it's collapsed into a centralized reality of major operators. Not only due to spam but as mentioned in this thread, running an email server today is not really possible because everyone thinks you're spam, and you have no way to prove otherwise.
Discussion
I actually run an email server. It is not that bad. A few times it went to spam folders of others.
I think in the future, when just starting users will need to gain reputation. During this phase the messages and pubkeys can be expensive. After reputation is built, messages (events) can be much cheaper or even free..
Some relays can be strict, then users can go to smaller relays to build reputation. Once the user acts well in the network (i.e. no spamming etc) then big relays can accept the events without PoW.