Expose message ID list from `ReferencesHeader`

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
70/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Quiet
Tech stack
python
Domain
backend

Research direction

Start by reading ReferencesHeader and the existing AddressHeader API, then inspect how ReferencesHeader currently exposes its private _parse_tree data. Make the message IDs available through the public header interface so the shown In-Reply-To and References usage works without private access, and verify that the exposed values preserve the existing message ID information.

Written by the indexing model from the issue text.

Description

stdlib topic-email type-feature

Feature or enhancement

Proposal:

When sending, parsing or validating mails which are replies to other mails it is necessary to access the message IDs from the In-Reply-To and References headers. These, however, are currently not exposed.

# Example use case
if "In-Reply-To" in msg and "References" in msg:
    [in_reply_to_token] = msg["In-Reply-To"].message_ids
    [*_, most_recent_reference_token] = msg["References"].message_ids

    if in_reply_to_token.value != most_recent_reference_token.value:
        raise ValueError(f"Error: References list end must match In-Reply-To message ID")

A workaround is to access the private _parse_tree property which does expose the message_id, though like any private property that should obviously be avoided. This is comparable to AddressHeader which already exposes addresses.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Dominant language
Python
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