In-Reply-To does not prove the parent exists

The header cites an id

In-Reply-To carries an angle-bracket id the creating software offered as the parent. References may list more ids. Those strings are claims in the header. They are not a search result.

A client may group messages into a thread from these headers. The grouping is a display. It does not prove you already hold the parent, and it does not prove anyone on your side wrote it.

A search is a second fact

Looking for that id in the mailbox you are allowed to search is a separate step. A hit means you found a stored copy in that search. A miss means that search found nothing. Neither result rewrites the header.

If you have not searched, the parent is unverified. Do not fill the gap by saying the thread is continuous.

What the case file can claim

Write the cited id and the search outcome: found, not found, or not run. Name the mailbox the search covered. Do not say the parent was verified when the search was not run.

Lab: a cited parent the mailbox does not hold

A lab message has In-Reply-To <lab-100@mail-out.example>. A search of the lab mailbox for that id returns no copy. A draft says the user already received the parent because the header names it, and treats the thread as verified.

The note quotes the cited id and says the lab-mailbox search found nothing. The header claim stays a claim. The thread is not marked verified.

In-Reply-To: <lab-100@mail-out.example>
search: lab mailbox
result: no copy
write: cited id and a miss
not: parent already received