Applied Brian Campbell <bacam@tardis.ed.ac.uk>'s mhn.defaults.sh patch:

    It appears that there shouldn't be quotes around the %s in the iso-8859-1
    charset entry; xterm passes the remaining arguments to the program, quoting
    them means that xterm thinks they're part of the program's name.

%s doesn't come from MIME headers, so not quoting it is safe.