Why would a reply to a message be classified as spam?

[hadaso]

Recently someone told me that a reply I sent to his message went into his spam folder (and that's why he responded days later).

Now the message he originally sent to me had this header:

Message-ID: <DB7PR01MB50775DCDFBC3CFEA5C5C145989F99@DB7PR01MB5077.eurprd01.prod.exchangelabs.com>

and my reply message had this header:

References:  <DB7PR01MB50775DCDFBC3CFEA5C5C145989F99@DB7PR01MB5077.eurprd01.prod.exchangelabs.com>

and the content of the last header proves beyond doubt that the message is a reply to the original message, so it is quite certain that it is not spam. Am I wrong in assuming this?

A Message-ID header contains a unique identifier of an email message, and is commonly generated using a random string so it cannot be forged by someone who doesn't have the original message. The References header contains the message-ID's of previous messages in a conversation (the message-ID header of the messaged being forwarded or replies to, and possibly additional message-ID's from the References header of that message).
So if an email system receives a message that has a message-ID string that it has created in the past, it should be able to recognize it and avoid classification of the reply as spam (certainly if also the recipient of the response is the sender of the message for which that Message-ID was generated).

Do spam classifiers actually use the References header to recognize responses to email that originated from their own system?

[janusz]

Maybe I'm missing something obvious, but why do you think that a reply to a message cannot contain spam?

[somdcomputerguy]

Well a reply can contain spam, or what the email provider might consider to be spam, but hadaso most probably didn't intentionally spam although his reply might've contained a link which could've been interpreted as spam. Or maybe the body of the message had some word(s) that might be 'spammy key-words'.

- Bruce

[hadaso]

Of course there is the theoretical possibility that someone replies to an email that they received and include some unsolicited advertising material. Does that make it spam?
Usually we use the word spam to describe the same (unsolicited) advertising content being sent to a multitude of recipients, not to (unsolicited) advertising material sent to one or a few recipients in response to previous communications they willingly participated in.

One type of spam that I did see in replies to email I sent is the advertising lines (and graphics) that some companies add to every outgoing email their employees send. It's annoying, but I still want to receive these emails since they contain information I need to get (responses to emails I sent), and the unsolicited advertising content only rides on the legitimate message i want to receive.

[janusz]

I never assumed that any spam filter does, or should, take into consideration whether the message is a reply.

[hadaso]

Originally Posted by somdcomputerguy

Well a reply can contain spam, or what the email provider might consider to be spam, but hadaso most probably didn't intentionally spam although his reply might've contained a link which could've been interpreted as spam. Or maybe the body of the message had some word(s) that might be 'spammy key-words'.

My point is that these "spammy keywords" or links or whatever are irrelevant when the recipients system can easily verify that the message is not part of a stream of millions of identical messages, but rather a response to a particular message from a particular user of their system. If they receive many copies of the same message to many of their users, they can classify it as spam. If they receive one copy of a message and it correlated to previously received spam (that is have "spammy" content) and there is nothing else differentiating it from any other message received, then it can be classified as spam. But if it is a single message received, and it is contain a clear sign that the recipient may be expecting to receive it (as it is a response to a message they sent), then I don't see how it can be considered spam, or how putting it in the recipient's spam folder serves the recipient's interest.
It is extremely unlikely that a real spam message would contain the message-ID of a message the recipient sent earlier. It is much more likely that the recipient expects that response, or at least would prefer to receive it.

[TenFour]

I can definitely confirm that just because you are replying to someone's email does not guarantee your reply won't end up in their spam or junk folder. Different email systems have very different ways of flagging spam. I have certain correspondents I have been in touch with for years that periodically go into my junk folder, and my email goes into their folder no matter what we do. The most recent memorable incident was my reply to someone contacting me with a job for my business. I replied and my email disappeared into his Junk folder.

[Bamb0]

Alot of my emails are sent to ppls spam folders and I dont know why!!

Happens alot on Yahoo.....

[SideshowBob]

I have heard of individual admins who have done something like this with SpamAssassin, they store outgoing IDs in in a database that's read during scanning. As it links outgoing mail with scanning it's more likely to be done on a proprietary basis which means it's hard to say who is doing it.

It's not completely safe either as it's possible to get IDs from mailing lists.

FWIW I used to configure my clients to use an anonymous looking free sub-domain in the message-id. If it showed-up in replies I took-off a few points in SpamAssassin, or handled it in sieve.