Why not use Perl's tools?

#!/usr/bin/perl -w
use strict;

# MDaemon msg to mailbox mail file converter
# Usage: msg2mbox *.msg > mail.mbox

die "usage: msg2mbox *.msg > file.mbox\n" unless @ARGV;

my @file = @ARGV;

FILE: for(@file) {
    @ARGV = $_;
    my @line = <>;
    my ($addr, $date);

    for(@line) {
        s/\r//g;
        if /^Encrypted\s+By\s+MDaemon/) {
            next FILE;
        }
        elsif(/^Return-path:(.*)/) {
            local $_ = join ' ', split " ", $1;
            tr/ //d;
            s/<(.+)>/$1/;
            $addr = $_;
        }
        elsif(/^Date:(.*)/) {
            local $_ = join ' ', split " ", $1;
            tr/,//d;
            # weekday day month year time adj -> weekday month day tim
+e year
            $date = join " ", +(split)[0, 2, 1, 4, 3];
        }
    }    

    $addr ||= "unknown\@uknown.com";
    $date ||= "Mon Jan 1 01:00:00 2000";

    print "From $adr $dt\n", @line;
}
[download]

Untested.

You should probably add a Lines: or Content-length: header if neither is present - or just let Mail::Box handle the gory details for you.

Makeshifts last the longest.