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One can only wonder whether the designers of modern e-mail clients are in league with spammers. Please allow me to explain.
I don't use a modern mailer; I use the same one that I've used for well over a quarter of a century. It's a simple, text-based mail client; it doesn't do anything fancy; it simply allows me to send and receive e-mail messages, i.e. blocks of text: paragraphs, sentences, words, characters. What e-mail was originally designed to do. If I want to send a picture or some other binary file, i.e. an attachment, I must use different software.
Why is that relevant to this question? Because that mail client also doesn't hide parts of the message including the e-mail headers. Not only the current headers but also any headers that the sender has not removed. Including those that any previous senders of a forwarded message have not removed. In a sense, it shows much of the history of that forwarded message includeing previous senders and previous recipients.
Because I see all those obsolete e-mail headers, see them taking up space and consuming network resources unecessarily, I confess, they annoy me. But that's not what this is about.
Within each of those headers is almost always a list of e-mail addresses. Those addresses will always be there unless that particular sender had the courtesy to use the "blind carbon copy" feature that is part of the e-mail standards.
Why do I blame the creators of modern mail clients? Because they apparently want to protect their users from seeing such ugly things as e-mail headers, new or old. I suspect that many, quite likely most, users of e-mail in this day and age, don't even know that there are such things as e-mail headers, much less that those headers typically include the e-mail address of everyone to whom the message was sent via either the "To:" line or the "Cc:" line of those headers. E-mail is sent by magic. How are they to know that there are such things as e-mail headers if their mail software hides those headers from them?
Not knowing that the message contains headers that the e-mail system has used to direct the message to each of its recipients, they would hardly think to try to delete the headers to protect the privacy of the others to whom their correspondent sent the message. Why would one think to delete something that they don't know is there?
Thus, each time a message, say a joke, is forwarded, it accumulates another e-mail header with yet another list of e-mail addresses.
Curious about just how many e-mail addresses some of these forwarded messages contain, I wrote a simple-minded shell script to extract them. Nothing fancy for I'm certainly not a highly skilled computer programmer. But even with my meagre skills I was able to extract long lists of e-mail addresses from such messages.
Even with this simple-minded script, I extracted well over four hundred e-mail addresses from a recent forwarded message. Yes, that's 400 e-mail addresses most of which belong to people I don't know. Four hundred e-mail addresses from a single forwarded message!
I get forwarded messages (jokes, amber alerts, legitimate virus and other malware warnings, hoaxes, ...) from quite a few people, only a few of whom bother to delete old headers before forwarding. If I were a spammer or if I approved of spam, just think of how many e-mail addresses I could collect and spam to or sell.
One assumes that modern mailers (aka mail user agents, MUAs, mail clients, ...), would make it easy to delete those headers IF the user knew they were there.
So, by hiding the headers from the user, creators of those mailers have, inadvertently or intentionally, created a situation where thousands (an understatement) of e-mail addresses are unnecessarily bounced round the Internet each day, Hell! each hour. Only the most recent set of addresses is necessary to distribute the message to the people it has been forwarded to; the old sets have done their job and are no longer necessary or desirable. Indeed, by using "blind carbon copy" as mentioned earlier, the only e-mail addresses that will appear in any given copy are those of that recipient and of the sender. The privacy of all other recipients will have been protected. But as long as the old headers are passed along with the forwarded message, any addresses in those headers are available to any recipient.