Hector Santos writes:
Can you elaborate what exactly they are not "adhering" to?
If I send you a body-part with content-type image/jpeg, that's what it
is. Some MUAs do this:
1. Tell the user there's a JPEG attachment and offer to show it.
2. When the user issues the "show attachment" command:
3a. Notice that the first six bytes (after base64 decoding) are "GIF87a"
3b. Choose to treat the body-part as image/gif instead of image/jpeg
3c. Show the attachment as a GIF file.
Sounds harmless to you? But if you do it you're deceiving your user.
Deception leads to misunderstandings and sometimes worse. What if the
body-part started with "MZ" or "%!" instead of "GIF87a"?
Another evil problem is to misparse boundary lines. If a MUA gets it
wrong, then a message can pass through the firewall, pass through the
virus checker, and by exploiting the MUA bug still cause much damage.
In the future, PLEASE try a google search rather than asking such
questions on mail-ng.