Version:           4.8 (using Devel) 
OS:                Linux

While not RFC-required, it is good practice to add a preamble to MIME messages right before the first boundary, which is displayed by MIME-incapable mail clients.

Usually the line "This is a multi-part message in MIME format." is given - at least Thunderbird and Outlook do. Here an example, how this looks like:
[...]
Envelope-To: test@test.com

This is a multi-part message in MIME format.

------=_NextPart_000_002F_01CCC242.8DE7A3A0
Content-Type: text/plain;
[...]

Reproducible: Didn't try

Steps to Reproduce:
1. Create a HTML Mail and send it to yourself.
2. Take a look at the source code of the newly received mail or look at it with a non-MIME client.

Actual Results:  
No message given

Expected Results:  
Should be given