How to send email to only of one the recipient?

Question

Is there a way to send email where receiver sees multiple recipient email addresses includes his, but in fact only send to one receiver himself?

Answer 1

Absolutely. During the sending phase you need to only talk to the MX server of the recipient and only specify them in the RCPT command. But I know of no MUA that can do so.

Answer 2

Yes, it is always possible to have the SMTP recipient list completely different from the "To:" or "Cc:" headers; the servers don't really care about the headers.

For example, that's how "Bcc:" addressing works (as there is no "Bcc:" header at all), but it can also go the other way and include headers that have nothing to do with the real recipient list.

In a SMTP conversation, it would look like this:

$ nc mailserver.example.net smtp
← 220 mailserver.example.net ESMTP Hello!
→ ehlo yourhostname.isp.net
← 250 mailserver.example.net
→ mail from:<KMC@nonexistent.org>
← 250 OK
→ rcpt to:<real-recipient@example.net>
← 250 OK
→ rcpt to:<another-recipient@example.net>
← 250 OK
→ data
← 354 Waiting for data
→ To: <fake-recipient@example.net>, <someone@else.tld>
→ Subject: Hello there.
→ Content-Type: text/plain; charset=utf-8
→
→ The thing about email is that you can spoof practically everything.
→ .
← 250 OK
→ quit
← 221 Bye

The addresses given in the envelope – rcpt – are the actual recipients. They will receive the message.

The addresses given in the header – To: – are only for display purposes. They are not used for sending.

When using the Unix sendmail interface, the same rule applies except the recipients are given in the command line:

$ sendmail real-recipient@example.net
→ To: <fake-recipient@example.net>, <someone@else.tld>
→ Subject: Hello there.
→ Content-Type: text/plain; charset=utf-8
→
→ One thing about email is that you can spoof practically everything.
→ CtrlD