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Re: Yahoo Mail Filtering

2010-07-21 11:47:02

On Jul 20, 2010, at 7:46 PM, Hector Santos wrote:

J.D. Falk wrote:

On Jul 20, 2010, at 8:09 AM, Hector Santos wrote:
It seem so random in what seems to be always accepting a transaction but 
intermittently delivered or not.
Are you talking about the 4xx replies?  If you check the text of the reply, 
there's usually a URL with a bit more information about the reason.  Same 
with 5xx, for that matter.

I have not seen temporary rejects.  All transactions accepted with 250. 
However, it has been noted by others that YAHOO does employ greylisting (in 
some form).  But I have not seen the temporary negative reply codes.

It's not the wikipedia definition of greylisting, but they do use 4xx replies 
to control the flow of inbound mail.

If you're saying the mail is just disappearing, it's possible that your 
users have configured their accounts to delete suspected spam instead of 
putting it in the spam folder.

The issue is that its intermittent and not consistent.  Send a test, with the 
recipient on the phone, he gets it.  Send another test, i.e, same message 
with "TEST #2" as subject, it never arrives. Send "Test #3" it may or not 
arrive, and so on. No pattern.

I remember hearing some years ago that Hotmail's SVM filter had learned that an 
empty message with a subject of "test" was spam.  They've probably fixed it 
since then, but it's a great example of how machine learning can lead to 
entirely logical but unexpected results.  (I'm fairly certain Yahoo! Mail 
doesn't use any Microsoft software, though.)

However, at some point, we tried the reverse, yahoo to non-yahoo, which 
arrived each time and then we try sending test messages again. For a moment 
there, it appeared to work, so we presumed there was a YAHOO user based 
"auto-white listing" to addresses the user sends mail to.  I jumped on this 
theory because we have that feature - local users can build an auto-white 
list just by sending mail to to their "social/business" network.

But then we try sending "test #4", "test #5" and so on and they done ever 
arrive or go into any folder.

Absolutely, no pattern, inconsistent "DISCARDING" and delivery of mail.

Often, filters will be reacting to a system-wide pattern that an individual 
user can't see.  To give an overly simplified and surely incorrect example, if 
a spammer were sending "test #4" all day, then your entirely legitimate "test 
#4" would get caught too.

That said, if the mail is being discarded due to system-wide (rather than 
recipient-specific) filters, I'd personally prefer to see a 5xx reply instead.  
But: their system, their rules.

I really hope someone with the yahoo story can explain it.

Like everyone else who writes anti-spam software, Yahoo! is not known for 
divulging their secrets.  Though I did work there for a time, everything I've 
written here could be gleaned from public statements and general knowledge of 
the techniques a large mailbox provider is likely to employ.

I was somewhat watching to see/hear if there was a related to valid DKEY/DKIM 
Brownie points or something now expected by YAHOO.  But all I know so far 
about that is that is required by BULK mail senders.  We are not bulking 
here. just individual single shot messages.

DKIM isn't required to send mail, bulk or otherwise.  Yahoo! does require DKIM 
for non-ISPs to subscribe to their complaint feedback loop, but doesn't 
automatically provide any brownie points.

Single shot messages are always the hardest to diagnose, since you can't see a 
pattern -- and if you send enough to create a statistically significant 
pattern, the spam filters will notice it too.

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