If you need to check that the email is being sent the way you think, set up a sendmail (or Postfix) alias that points to a file, and send the message concerned to that address. You'll then get a file containing the message exactly the way sendmail (or postfix) received it. No email client will have got anywhere near it.
You can then examine that file with Emacs or with any hex dumping tools you prefer to make *sure* the body is what you think it is. You could even upload a copy of the file to new-file-storage for others to take a look at, too.
As Don said, if you are sending nice normal standards-compliant plain text email, and email client programs are messing with it, that is a problem that you can't solve at the sending end of things at all.
