if ($mail_id =~ /\A(box1|box2|box3|other1|other2)*\.(yourcompany|othercompany|elsewhere)\.com\z/ ) {
  print "matched\n";
}

##</code><code>##

if ($mail_id =~ /\A(box1\.yourcompany\.com|box2\.yourcompany\.com|box3\.yourcompany\.com|other1\.othercompany\.com|other2\.elsewhere\.com)\z/ ) {
  print "matched\n";
}