When PRPC tries to send an email and it cannot be delivered. Is there a log or report that contains these undelivered emails. Also, is there a log or report that has successful emails. Sometimes the email address is malformed and sometimes the email address becomes invalid and we need to find these exceptions.
If the email id is not is malformed, you should see exception in App server and Pega log file. In case the recipient email id was invalid, the bounce will reach to your mail server not to Pega, unless you are scanning the reply-to email box as well.
You can control the issue of malformed email ids by validating the email id on entry (via edit validate or post activity). For handling bounces, you need to either do it outside Pega (on mail server or in a mail client) or configure to handle emails landing in the reply-to mailbox.