When I used muttprint (with print to file, for checking) on a mail with δ and ε characters, these characters were silently missing from the generated .ps file. With debug output, I can see errors: ! Undefined control sequence. \u-default-949 #1->\textepsilon ! Undefined control sequence. \u-default-948 #1->\textdelta
I think that the easiest solution would be to use xelatex and a font that the required characters (DejaVu Sans Mono seems one of the most complete fonts). I've attached a patch that does this (mainly for testing), but I haven't tried in non-UTF8 locales (I suppose that the input should be converted to UTF8 or \XeTeXinputencoding should be used) and the font is hardcoded in the patch.