Hi,
We have recently found that the issue is most probably caused by the bounced emails (delivery failure/status notification) with non-standard encoding specification.
Failed delivery (NDR, DSN, NDN) messages are commonly sent with
Content-Type: text/plain; charset=iso-8859-1
However, the emails that cannot be processed have
Content-Type: text/plain;
charset="iso-8859-1"
You can see the difference - the new line won't be an issue probably, but the double quotes cause a problem, because it's not standard and when the
OpenPOP library we use processese this, it throws an error (as it's more strict as per standard following).
The other bounced emails should be processed correctly (if you run 6.0 with some later hotfix), but the skipped emails are not deleted from the server so they are processed repeatedly everytime.
To fix the issue now, you will need to periodically check the DNF emails in your server and delete the older ones...
We will be able to adress this in the next version, where some additional logic may be able to detect those nonstandard encoding specifications.
Regards,
Zdenek