To simplify the explanation here the RCPT command using the verb “to:” is used to direct the email to a specific address. In the last mile the email will be received by the addressee but only the "to:" header if present will be accessible. If it is missing you get "to: Undisclosed recipients:;" and if it is set you get whatever it says. Clearly you can use a different email address there which will generate in some cases a heck of confusion ;-). You can confirm this yourself just by using telnet as usual for SMTP:
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# WHEN "RCPT TO:" = "TO:" (to: <me@sample.com>) | |
MAIL FROM:me@sample.com | |
250 2.1.0 Sender OK | |
RCPT TO:me@sample.com | |
250 2.1.5 Recipient OK | |
DATA | |
354 Start mail input; end with <CRLF>.<CRLF> | |
subject:Using RCPT TO: = to: | |
to: me@sample.com | |
# WHEN "RCPT TO:" != "TO:" (to: <who@et.com>) | |
MAIL FROM:me@sample.com | |
250 2.1.0 Sender OK | |
RCPT TO:me@sample.com | |
250 2.1.5 Recipient OK | |
DATA | |
354 Start mail input; end with <CRLF>.<CRLF> | |
subject:Using RCPT TO: != to: | |
to: who@et.com | |
# WHEN "TO:" IS NOT USED (to: Undisclosed recipients:;) | |
MAIL FROM:me@sample.com | |
250 2.1.0 Sender OK | |
RCPT TO:me@sample.com | |
250 2.1.5 Recipient OK | |
DATA | |
354 Start mail input; end with <CRLF>.<CRLF> | |
subject:Using no to: | |
. |
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