By changing one line in the current (second) version of the patch, I was
able to get it working with roundup 1.4.18 (I'm using an old version as
shipped with Ubuntu precise, but I presume the patch applies cleanly to
later versions too):

+            mailer = subprocess.Popen([sendmail, "-t"],
stdin=subprocess.PIPE, stderr=subprocess.PIPE)

In other words, use the -t parameter to sendmail (in my case, sendmail
was esmtp) as specified in msg4906.