Except your client does not live in a bubble. It needs to communicate and send emails to other clients. In order to combat spam, many popular email providers rank messages using an array of criteria, coming from a known and well publicised email provider is one such criteria and not coming from a well known provider is one such criteria that redirects your email to many people's spam folders. So the question remains, how do we ensure that nostr relays don't start to use exclusionary metrics to avoid spam and abuse?

Nostr has many issues with the decentealizatuon concept. And if we are honest Nostr is already some kind of centralized as we have not thousands of relays. You can solve the issue with spam with paid relays but then the whole concept is senseless

You can install it but it's a mission to maintain and ensure other large centralised providers accept your email (gmail and outlook).

I know this article but still you can install your own SMTP and use it