mail Fetch Bounces errors
My sense is that this is more likely to be a hosting/configuration issue than a Wordpress issue. Though it could of course be a hosting issue that only affects Wordpress sites :p.  Did you try testing on http://wpmaster.demo.civicrm.org/?

'Empty mail protocol' is from the more helpful end of the error message spectrum.

I would first check on this page civicrm/admin/mailSettings?reset=1 that the protocol is set for the processor you are using for Bounce processing.

If and when you have ruled out any issues there, it would be time to start digging a little deeper...



Hopefully you don't have to read this far, but just in case...

Depending on your willingness to jump into the code case, the next step would be to look through the code of the Fetch Bounces scheduled job. https://github.com/civicrm/civicrm-core/blob/master/api/v3/Job.php#L356 passes of to https://github.com/civicrm/civicrm-core/blob/master/CRM/Utils/Mail/EmailProcessor.php quite quickly.

It looks like your error message is being generated here: https://github.com/civicrm/civicrm-core/blob/master/CRM/Utils/Mail/EmailProcessor.php#L165 and here: https://github.com/civicrm/civicrm-core/blob/master/CRM/Mailing/MailStore.php#L57.

So the question is, why isn't $protocols[$dao->protocol] set on https://github.com/civicrm/civicrm-core/blob/master/CRM/Mailing/MailStore.php#L56?

Happy to expand on this answer if necessary.