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Nov 3, 2021

MAIL FROM and RCPT TO vs. the To and From headers

The difference between the email envelope and message headers when it comes to senders and recipients.

Apart from the message body, every email message includes the so-called envelope and message headers. In fact, both the envelope and the headers contain the information about the sender and the recipients of the message, and they do not always have to match.

The difference between the two sets is as follows. The recipient addresses specified in the envelope (using the RCPT TO command) are the actual addresses that the sender wants the email to be delivered to. Upon delivery of the message, each recipient won't be able to figure out what other addresses it was meant to be delivered to. The addresses specified in the To message header however, are all included with the email message, and everyone receiving the message sees that same header in full.

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See also
Eckher SMTP Testing Tool
Test SMTP server communication using Eckher's email testing tool.
Receiving email with Node.js
How to receive email in Node.js?
cid URLs for email developers
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