Bouncing usually result in that the recipient is not available. So it seems, that are incoming mails and you activated the option to check the recipient, so maybe your mail system does not support verified recipients. So you should either enable this option on your destination system or disable the check in PMG, which may result in more spam comming in as some spammers just test combinations of addresses, if there is a recipient, your destination system will then send NDR, which result in bad behaviour as you could be recognized as backscatterer.