[v6z] Re: Yahoo! Mail Issue

Scott Howard scott at doc.net.au
Sat Apr 16 21:09:46 UTC 2011

On Tue, Apr 12, 2011 at 2:22 AM, Nathanael C. Cariaga <
nccariaga at stluke.com.ph> wrote:

> ps.  I'm just wondering why yahoo doesn't inform their users that the email
> that they sent was blocked because of their servers were listed in a
> blocklist (inspite that the server is able to return a correct reject code
> 550)

Because 550 is NOT a valid response code at that stage in the conversation.
According to the RFC, the only two valid responses to an initial connection
are a 220 or a 554.

Even then, RFC 2821 doesn't make it clear if a 554 on initial connection
should be considered a fatal error at the message level, and as a result
most mail servers will consider it a temporary failure and will re-try to
send the message multiple times even after getting a 554 (and especially
after getting an invalid 550).

As someone else has already pointed out, the solution is to return the 5xx
response after the rcpt to, not at the initial connection.

On 4/12/2011 3:33 PM, Matthew Petach wrote:
>> -bash-3.2$ telnet qc.stluke.com.ph 25
>> Trying
>> Connected to qc.stluke.com.ph.
>> Escape character is '^]'.
>> 550 Blacklisted: Blocked - seehttp://
>> www.spamcop.net/bl.shtml?
>> Connection closed by foreign host.
Closing the connection immediately after sending the 5xx is also not RFC
compliant. You "MUST" give the client the opportunity to close down the
connection with a quit command.


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