If bounce address is not working, no emails are sent and no warning or error is generated.

If you set-up civi-mail with a working bounce email address which then stops working, civi-mail just stops sending emails and does not give any indication of why (like a pop-up warning saying that your bounce address is no longer available). This happened to me recently where someone deleted the bounce address. It took me a huge amount of time to figure out why Civi-Mail just suddenly stopped sending emails. No warning, nothing in any logs... nothing anywhere. There really should be some kind of warning...