Issue #11388 has been reported by Evgeniy Serykh.

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Feature #11388: SMTP Service Extension for Delivery Status Notifications
https://bugs.ruby-lang.org/issues/11388

* Author: Evgeniy Serykh
* Status: Open
* Priority: Normal
* Assignee: 
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Considering https://tools.ietf.org/html/rfc1891 and http://www.sendmail.org/~ca/email/dsn.html

TD;DR 
SMTP session via Telnet:
`R: 220 Pure-Heart.ORG SMTP server here
S: EHLO Pure-Heart.ORG
R: 250-Pure-Heart.ORG
R: 250-DSN
R: 250 SIZE
S: MAIL FROM:<Alice / Pure-Heart.ORG> RET=HDRS ENVID=QQ314159
R: 250 <Alice / Pure-Heart.ORG> sender ok
S: RCPT TO:<Bob / Big-Bucks.COM> NOTIFY=SUCCESS,DELAY ORCPT=rfc822;Bob / Big-Bucks.COM
R: 250 <Bob / Big-Bucks.COM> recipient ok
S: RCPT TO:<Carol / Ivory.EDU> NOTIFY=FAILURE ORCPT=rfc822;Carol / Ivory.EDU
R: 250 <Carol / Ivory.EDU> recipient ok
S: RCPT TO:<Dana / Ivory.EDU> NOTIFY=SUCCESS,FAILURE ORCPT=rfc822;Dana / Ivory.EDU
R: 250 <Eric / Bombs.AF.MIL> recipient ok
S: RCPT TO:<Fred / Bombs.AF.MIL> NOTIFY=NEVER
R: 250 <Fred / Bombs.AF.MIL> recipient ok`

Now **Net::SMTP#rcptto** looks like this:
`
  def rcptto(to_addr)
    if $SAFE > 0
      raise SecurityError, 'tainted to_addr' if to_addr.tainted?
    end
    getok("RCPT TO:<#{to_addr}>")
  end
`

It's impossible to use SMTP Service Extension for Delivery Status Notifications in current implementation?



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