On 20/03/2020 12:43, Martin Gregorie wrote:
> On Fri, 20 Mar 2020 10:47:38 +0000, The Natural Philosopher wrote:
>
>> On 20/03/2020 10:02, Martin Gregorie wrote:
>>> Its well-known that the From: header is not used at all to transfer
>>> mail from sender to receiver - returned bounces are sent to the
>>> Reply-To address.
>> No, to the envelope from address.
>>
> Regardless, the point I was trying to make is that the From: header is
> not used by the process of transferring mail from sender to recipient and
> is displayed by the receiving MUA for information only.
>
> So it can be, and is, used nefariously by spammers and other lowlife.
>
>
No, the From: address is simply a header until you try and respond to an
email, when it is, after 'reply to:', the default recipient (long with
any cc: lists.

So it is not just displayed for information.

For clarity, the SMTP protocol is:
Helo  (can be anything)
Mail From 
RCPT to 
data
which is where the headers are sent, followed by the message parts.

That is, the SMTP exchange has nothing to do with the headers BUT once
delivered, the headers are what the MUA uses to respond




--
“when things get difficult you just have to lie”

― Jean Claud Jüncker

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