To send an email on Linux such that the recipient A receives the email, but the To: field in the email header shows B (so A sees "To: B" when they open the email), you can use the sendmail command or a command-line email client like mail or mutt with custom headers.

Using sendmail (most common method)

You can use the following command:

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echo "This is the email body" | sendmail -f your_email@example.com -- A@example.com

But to set the To: header to B, you need to manually construct the email headers. Here’s how you can do it:

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(
  echo "To: B@example.com"
  echo "Subject: Your Subject Here"
  echo ""
  echo "This is the email body"
) | sendmail -f your_email@example.com A@example.com

Explanation:

• The To: header is set to B@example.com, so A will see "To: B" in their email client.
• The actual recipient is A@example.com (passed as an argument to sendmail).

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Using mail or mutt

If you have mail or mutt installed, you can use:

With mail:

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echo "This is the email body" | mail -s "Your Subject Here" -r your_email@example.com -S "to=B@example.com" A@example.com

With mutt:

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echo "This is the email body" | mutt -e "set from=your_email@example.com" -s "Your Subject Here" -- A@example.com -H - <<EOF
To: B@example.com
Subject: Your Subject Here

This is the email body
EOF

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Note:

• Replace your_email@example.com, A@example.com, and B@example.com with the actual email addresses.
• This method works because the To: header is just a header and does not control where the email is delivered. The actual recipient is specified as an argument to the command.