I’m fairly certain NameCheap Private Email isn’t a transactional mail service (I can’t find anything to say it is?). Discourse recommends a transactional mail service be used. You may encounter problems like this if you use things like NameCheap Private Email. If it works for you, then 
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The transactional mail providers I suggested above are mostly free (except Amazon SES, but it’s still quite cheap). Brevo is 300/day, IIRC Mailgun is 100/day.