| Internet-Draft | SRDS | March 2022 |
| Brotman | Expires 1 October 2022 | [Page] |
Define a method by which an SMTP receiver can immediately notify a sender that their message is suspected to be spam, though is still being accepted.¶
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Today, a typical SMTP transaction ends with a "250 OK" and the message is then inspected by the receiver and routed to the inbox or spam folder as appropriate. In some cases, it may be desirable for the receiver to provide in-line feedback. This could be done via the SMTP response. This document proposes a new response code to receivers to optionally use to provide this feedback.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].¶
In the email ecosystem, there exist a few mechanisms by which a receiver or recipient can provide feedback to the sending entity, such as Feedback Reports [RFC5965] or Reputation portals. Historically, these have been out-of-band or delayed. In some cases, this is more than sufficient, and properly conveys information to the sender. Given the out-of-band nature, these do not allow for immediate feedback to the sender that their messages may be construed as undesirable by the recipient or some automated system within the platform. By providing this feedback to responsible senders, they may be able to more immediately use that feedback to remediate the responsible party. In the case of an Email Service Provider or Mailbox Provider, this information could allow them to cease delivery before incurring further risk to recipients.¶
This document adds the 259 reply code, and defines this as a signal to
the sender that the receiving system believes the attempted message to
be malicious or undesirable in some way. The receiving system intends to
accept the message, and then deliver this to the spam folder of the
recipients. The code SHOULD only be used when the receiving system has
already determined the message has been determined to be undesirable. This
implies that the receiving system will have evaluated various parts of the
message before fully accepting the message. The 259 response code MUST only
be used after the . has been used to indicate the end of the message.¶
A sample response could be:¶
259 OK - Delivering to spam folder¶
... C:DATA S:354 OK C:From: Bob@example.com C:To: Alice@example.net C:Subject: Sample spam message C: C:XJS*C4JDBQADN1.NSBN3*2IDNEN*GTUBE-STANDARD-ANTI-UBE-TEST-EMAIL*C.34X C: C:. S:259 OK - Delivery to spam folder C:QUIT S:221 mailhost.example.net closing connection¶
It may be desirable for the receiver to disclose some sort of value to denote assuredness of the malicious verdict. So consider a scale of 0-100 where 0 is a legitimate message and 100 is definitively spam, perhaps the receiver may disclose this as such:¶
259 OK - Delivery to spam folder (85/100)¶
Doing so could aid the sender in determining the strength of this signal. Other options could include more detailed information, though the receiver should reference the Security Considerations before doing so.¶
Understandably, there are some questions about what benefit this can provide on both the sending and receiving side. This should be considered from both sides and understand that this could allow for a more collaborative interaction.¶
Receivers could realize some benefit from deploying this signal. The signal could help deter senders from continuing to send messages that you will only filter into the spam folder. This could help to reduce volume into your platform, reduce storage requirements, and generally reduce load on your platform. In the case of an attack, the sender could see this signal and terminate the offending sending account.¶
As a sender, using this information can help you understand when your messages are spam. Depending on the sources of these messages, that could imply that the sender has a bad list of recipients, a malformed message, or truly is spam. An additional possibility is that the sending account is compromised or has been created fraudulently for the express reason of attempting to send undesirable messages.¶
When providing information to a sender, care should be taken to give information to reasonable and reliable entities. Using this code to inform an intentionally malicious sender may have an undesirable effect. The malicious party could attempt to more easily circumvent a receiving party's anti-spam mechanisms. By delaying the 259 until the end of DATA, that allows for some obfuscation as to which data points caused the receiver to believe the message to be undesirable.¶
A receiver should take precautions to utilize the 259 response only when speaking with parties they believe will use that data responsibly. This could be accomplished via a number of methods or ACLs. These methods could include IP, CIDR, PTR, DKIM/SPF, ASN, internal reputation mechanisms, and so on.¶
Additionally, as the message resulting in a 259 response is accepted by the remote system, there should not be an NDR to notify the sender that their messages have been regarded as spam.¶
An associated Enhanced Status Code will be requested. The code would then be listed in the table at IANA, with a reference to this document.¶
https://www.iana.org/assignments/smtp-enhanced-status-codes/smtp-enhanced-status-codes.xhtml¶