An envelope lists a sender and one or more recipients . In more complicated situations, though, the envelope addresses may differ from the header addresses. When a message is delivered to several people at the same location, the sender doesn't have to photocopy it. The envelope recipient addresses can be taken from the command line or from various header fields, depending on the options to qmail-inject .
Similar comments apply to sendmail .
When a message is transferred from one machine to another through SMTP, the envelope sender is given in a MAIL FROM command, the envelope recipients are given in RCPT TO commands, and the message is supplied separately by a DATA command. To to detect mail forwarding loops. Note that, if the header doesn't have any recipient addresses, sendmail will move envelope recipient addresses back into the header. This situation occurs if all addresses were originally listed as Bcc , since Bcc is automatically removed. When sendmail sees this, it creates a new Apparently-To header field with the envelope recipient addresses.