One or more spammers have been conducting a Joe Job, forging email addresses in the From: field of their spam to make it appear that messages are being sent from the ptb.org domain.

I do not have anything to do with this spam and am suffering from the same inconvenience: I get many error messages from ISPs (user not found, mailbox full, spam detected, etc.).

If you wish to make a complaint, please forward the entire message (with headers) to your ISP. Their staff will know how to interpret the routing information in the header and potentially block the real source of the spam.