Which is best way to add footer text in email?

Question

My target is to modify MIME contents & add footer note in email, without changing it's behavior.

If email content content type is text/plain then I'm updating email content type to multipart/mixed as below:

Content-Type: multipart/mixed;
Boundary="--=_SAMPLE_MIME_BOUNDARY_"

email original data

----=_SAMPLE_MIME_BOUNDARY_
Content-Type: text/plain;
Content-Transfer-Encoding: 7bit

!-- Sample footer note --!
----=_SAMPLE_MIME_BOUNDARY_--

And if email content type is by default multipart/mixed then I'm updating email contents are as below:

Content-Type: multipart/mixed;
boundary="----=_SAMPLE_MIME_BOUNDARY_"

------=_SAMPLE_MIME_BOUNDARY_
Content-Type: text/plain;
Content-Transfer-Encoding: 8bit

email data

------=_SAMPLE_MIME_BOUNDARY_
Content-Type: application/octet-stream;
name="test_attachment"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="test_attachment"

<<attachment data>>

------=_SAMPLE_MIME_BOUNDARY_
Content-Type: text/plain;
Content-Transfer-Encoding: 7bit;
Content-Disposition: inline;

!-- Sample footer note --! 
------=_20160127171644_62291--

But when I update email body then Outlook treat sample note as an attachment. And in case of Thunderbird when I click on forward email it will treat footer as an attachment.

So my questions are, Which is best way to add footer text in email? Am I doing right? Suggest changes/pinpoints if anything wrong.

Answer 1

The MIME structure of your message is wrong. It needs to be

multipart/mixed
    multipart/alternative
        text/plain
        text/html
    application/octet-stream