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Pop Quizz: DNS/SMTP

Given

> dig turbojet.com.hk MX

;; QUESTION SECTION:
;turbojet.com.hk. IN MX

;; ANSWER SECTION:
turbojet.com.hk. 3083 IN MX 10 smtp.turbojet.com.hk.
turbojet.com.hk. 3083 IN MX 20 relay.incnets.com.

and

> telnet smtp.turbojet.com.hk smtp
Trying 203.80.224.23...
Connected to smtp.turbojet.com.hk.
Escape character is '^]'.
220 CheckPoint FireWall-1 secure ESMTP server
ehlo iota
250-Hello iota, pleased to meet you
250 SIZE
mail from: <>
250 <> ... Sender ok
rcpt to: <e+trial@turbojet.com.hk>
550 Mailbox unavailable.
quit
221 Closing connection

describe why an email from an SMTP-compliant mail transport agent will still reach the respective mailbox.
Comment on the efficiency of the current setup, and give a recommendation for improvement.
Bonus question: Analyse the email-address with regard to the so-called user + detail syntax of a popular MTA — can you make an assumption which other email addresses will likely be valid?



Avatar: Eike M. Lang

Re: Pop Quizz: DNS/SMTP

Hm, a compliant agent would also try the second (in terms of order) MX entry wouldn't it? Assuming that the second MX will accept mail for valid users, the message will get there.

I'll have to pass on the efficiency and bonus questions, though :(

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