Thursday, July 03, 2014

On SMTP: RCPT To: command verb versus to: header

Ever wondered why you got an email with "to:" being some other address and not yours? Perhaps you got an email stating "to: Undisclosed recipients:;", why? you might have asked.

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:
# 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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