On my cluster (let's posit that its domain is example.com), this means that completing jobs all send a report to notexistent@example.com, which bounces. This appears to be set in job.c and I can't figure out how to turn it off. The system administrators have asked me not to submit any more jobs until I have a fix somehow.
In job.c change:
req->mailUser = fsd_strdup( "notexistent" );
to
req->mailUser = fsd_strdup( "nobody" );