Will consolidation of email providers point to consolidation of fediverse instances?

This is useful to note. The email protocols are open and one can chose to host their own email server that—at a protocol level—can interoperate with any other. At a practical level, though, there is now service requirements (spam filtering) and policy choices (only accepting mail from known good sending servers) that limit the reach of a new, bespoke mail server.

What would the equivalent of an email spam problem look like on the fediverse?