ZFilter takes the following steps...
- If it ever can't lock or open a file, it aborts opening it. - If it was trying to write to a mail folder, it gives up and tries to write to the inbox instead.
- If it can't write to the inbox, it tries to save the message as a file in your home directory called "dead_letter.####" (where #### is the process ID for ZFilter at the time, a semi-random number)
- If it can't save the dead letter file, it sends the message (with an error report) to the local postmaster, sends just the error report and message headers to the postmaster at the sending site, then sends the message (no error report) back to the sender, and mails you an error report and message header in hopes that it will eventualy get through. (If it encounters the same error when trying to save the error report, it just keeps sending it back to you)