On Fri, 20 Mar 2020 13:03:53 +0000, The Natural Philosopher wrote:

> 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.
>
The fact that you can reply to it only occurs because your MUA displayed
it in such a way that you can reply to it. It can be plain wrong: how
many times have you seen a sender displayed by your MUA as:

From [Close Friend ]

You haven't? Then one of the following applies
(a) your MUA isn't configured to show the address after the display-name
(b) your MUA can't/won't show the from address, so ditch it and get a
    better one
(c) you haven't been watching out for spam


> So it is not just displayed for information.
>
> For clarity, the SMTP protocol is:
> Helo  (can be anything)
> Mail From 
>
envelope sender is NOT the From: header and nothing says that there has
to be a From: header or that irs content has to be the same as the
envelope-from content

Same goes for the Return-address: and Reply-To: addresses though, because
they are automatically set from the sender's account name and server
domain, they are more likely to be correct.

> 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


--
Martin    | martin at
Gregorie  | gregorie dot org

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