email bounce processing not working. site on AWS, using Zoho email
Is your bounce address set up properly in Administer menu > System Settings > Mail Accounts?  Documentation is available here.

If you're good there, make sure you can log into the bounce processing account with the credentials you've stored.  Also, what protocol have you set on that page?  Is IMAP an option for you to use?  If so, CiviCRM will store emails that it's processed in a subfolder on the server.  It makes it easy to look and see whether Civi is doing what you expect.

Finally, if you aren't committed to Zoho, there are many third-party mailers, some of which offer as many as 100,000 messages free/month.  Many have an extension for CiviCRM, much easier to set up than traditional bounce processing.  Check out this blog post that includes a link to a CiviMail calculator.