I think that you should bounce back mail and this is my experience that I
would like to share.
My Postmaster set up a log named "from" so that I can see the log of who has
contacted me no matter where it is delivered. So, I can see when stuff has
been bounced back and when stuff is sent to dev/null.
When I DO NOT bounce back, I am often contacted for a very long period of
time. One lady recently contacted me a number of times in one day and I
could see it going to dev/null so I didnt know what her mail said.
But after I put her on Bounceback, she never contacted me again. I have
noticed this other times. I think that it "proves" to the harasser that you
are not reading their mail and they give up instead of keep sending you mail.
Since some of the people who have harassed me have sent me several e-mails
every day for months, I think that it might actually SAVE bandwidth.
One exception is that once when I was e-mail bombed I did put it all in one
big file for the police and did not bounce back.