Typically, most email clients create an email header when forwarding, eg… here are the ones that Thunderbird would use…
References: [email protected]
X-Forwarded-Message-Id: [email protected]
WIthout a header to reflect that it is forwarding an email, it can appear as a fake forward, and trip up spam filtering rules…
We don’t include the References header for forwarded messages, but we do for replies, since that’s how it is specified in RFC 5322:
The “In-Reply-To:” and “References:” fields are used when creating a reply to a message. They hold the message identifier of the original message and the message identifiers of other messages (for example, in the case of a reply to a message that was itself a reply). The “In-Reply-To:” field may be used to identify the message (or messages) to which the new message is a reply, while the “References:” field may be used to identify a “thread” of conversation.
Anything starting with X- is by definition a non-standard header, so X-Forwarded-Message-Id is not required. And we would not expect any spam filter to actually take the X-Forwarded-Message-Id header into account either.
In fact, there is at least one bug report filed for Thunderbird that requests the thing not to be sent, because with S/MIME the information is leaked in the non-encrypted envelope, so is a security risk. And also when the user explicitly deletes the "Fwd: " from the subject, the header still indicates the forward which may cause expected behavior.
But apparently Thunderbird does insert those headers for forwarded messages, though it is not really the correct thing to do. We are unlikely to adopt that without some compelling reason. Also, adding the References header for forwarded messages would result in different conversation grouping and other unintentional side effects.
Just used Thunderbird as an example, but for the record, most email clients do include a reference header of some type, and it helps to determine if it was a real forward, or a forged forward, and many filtering softwares do have rules to detect forgeries. But thanks for your thought out reply, For the record, many other email clients follow a similar standard, they create a new Message-Id for the thread, but use the In-Reply-To and References headers, when forwarding. Suggest that this be still considered. And generally, forwarded messages won’t mess up threads 