What is an SMTP envelope?

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SMTP borrows the postal term envelope, ie the email message is put in an envelope that nobody can see.

The envelope is defined during a SMTP transaction by the commands:

• MAIL: that define the sender address (not the from address from the message)
• RCP: that defines the recipient addresses
• DATA or BDAT: that defines the message

The envelope is delivered once the SMTP server has received it.