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8bit and binary are valid values for the Content-Transfer-Encoding header (here is a nice summary on SO).

However, trying to figure out which one was the most suitable for my needs, I encountered the following notices :

Binary encoded messages are not valid Internet messages.

and

Because not all Message Transfer Agents (MTAs) can handle 8bit data, the 8bit encoding is not a valid encoding mechanism for Internet mail.

Digging a bit I found out these warnings likely origin from Microsoft documentation.

What does it actually means ? Should one avoid these values ?

NB : It is not clear to me what the quoted "Internet messages" term specifically refers to. For my purposes, I am concerned only with multipart emails.

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Skippy le Grand Gourou
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    You have to differ between different services. What do you actually refer to by "Internet Message"? MTAs transport _email messages_, for that indeed only 7bit encoding can be considered absolutely safe. A "Content-Transfer-Encoding" header typically is used with the http and https protocols, so for _web service_, which are something totally different. For that certainly different encodings are valid. – arkascha Mar 22 '17 at 09:47
  • As said: traditionally only 7bit encoding is defined for email messages (SMTP protocol). Most MTAs do support 8bit encoding these days, but you simply cannot rely on it. That is for example why most email client applications have a settings option to "allow to send 8bit encoded messages". – arkascha Mar 22 '17 at 10:00
  • Ok, with the help of your comment I was able to find that 8bit and binary values are not suitable for SMTP, *unless* the SMTP server supports the 8BITMIME and BINARYMIME extensions, respectively. I run my own SMTP server (exim4), which seems to support at least the former. In this case 8bit messages are valid, right ? Or should I expect issues with recipients old email clients ? – Skippy le Grand Gourou Mar 22 '17 at 14:15
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    No, your conclusion is wrong. Yes, your own SMTP server will (hopefully) accept and correctly handle such messages. However that says _nothing_ about the way some _other_ SMTP server acts on such messages. Keep in mind that email messages typically go through multiple SMTP servers in a row... – arkascha Mar 22 '17 at 14:57
  • Bottom line: strictly formal you should limit yourself to 7bit encoding. But in real life no such limited support exists any more. It should be safe to send 8bit encoded messages. – arkascha Mar 22 '17 at 14:58

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