How are you accessing the email-------on a computer using only the webmail and a browser? Or in a desktop email client like Outlook or Thunderbird? Do you have it enabled on a mobile device? Do you mean that you have the deletion set to 30 days in Comcast or on a client or what?
Try setting the time to something shorter like 7 days. Then close out of the account and sign back in again. Does anything change?
And just a heads-up; if you are still in the old email format, at some point you will be migrated to the new one, and you will have no editable schedules. With the exception of the Spam and Trash folders, all others will be kept indefinitely until you delete them.
I did what you suggested. I changed the deletion schedule to 7 days, logged out of Xfinity and closed Firefox.
I opened everything up and nothing changed. I still have trash that is from September of 2016 along with other email.
I suspect that I am using the old version of the email program.