SMTP is quite reliable compared to Nostr.
I've never had emails vanish out of my Inbox. Finding emails I've sent has a 100% success rate. If a message doesn't reach its destination, I get notified.
I don't see the flaws there. I think the only thing missing from the protocol itself, is a way to bounce emails back to the sender if they're unwanted (spam), or at least "trash upon arrival".