Trouble sending emails

That’s why the standard install includes a list of recommended mail providers. If you want to deviate from those recommendations then you also accept the additional technical complexity that it introduces. Not all mail servers behave the same, it’s not the place of Discourse to address that.

The summary of the above is you misconfigured Zoho. Discourse can’t see the configuration there, and assumes you’re providing it with correct info. Had you rerun ./discourse-setup, it would have asked you again for the sending address, which you would have entered incorrectly due to this misconfiguration.

We can save users from a lot of problems, but rarely from themselves. Next time around I would suggest focusing on the more likely aspects and double-checking the details on external systems, before insisting that the issue lies in code that’s being utilized to serve tens of millions of users. It will likely get you to your answer far quicker.