Why does changing the email subject in Outlook create a new thread?

If you change the subject line of an email in an Outlook conversation, Outlook treats the modified message as the start of a new, separate conversation. This means the original thread is split: messages with the old subject stay in one thread, while messages with the new subject appear in another.

Because Tekst relies on the conversation ID that Outlook assigns to group messages into threads, a subject change in Outlook results in:

Separate threads in your message feed - the original conversation and the renamed one show up as two distinct threads.
Separate processing runs - each thread is processed independently by your AI models.
Multiple automation runs - automations that trigger on incoming messages will run separately for each thread, potentially causing duplicate actions such as repeated replies, label assignments, or ticket creation.
Split conversation history - context from the original thread is not carried over to the new one.

To Tekst, the renamed thread looks like a brand new email arriving, because it carries a new thread ID. This is often what confuses customers: the "new" thread runs through your automation again, exactly as if a fresh email had come in.

Why this happens

Outlook uses the email subject line as a key component of its conversation ID (also called the thread ID). When you modify the subject, Outlook generates a new conversation ID for that message and any future replies to it. This is a limitation of how Outlook handles email threading - it is not something Tekst can override or work around.

The original conversation does not move into the new thread. Instead you end up with two parallel threads:

The original thread keeps the old subject and its original thread ID.
The renamed thread gets the new subject and a brand new thread ID.

Thread ID vs Internet Message ID

It helps to understand the two identifiers involved, because they are easy to confuse and they answer different questions.

Internet Message ID

The Internet Message ID (the Message-ID header defined by the email standard) is a globally unique identifier assigned to a single email message at the moment it is created:

Every individual message has its own Internet Message ID.
It never changes for that message and is never reused.
It identifies one specific email, not a conversation.
Replies point back to earlier messages through related headers (In-Reply-To and References), which is the standards based way mail clients link messages together.

Think of the Internet Message ID as the fingerprint of a single email.

Thread ID (conversation ID)

The thread ID (Outlook and Exchange call this the conversation ID) is an identifier used to group multiple messages into one conversation:

It is shared by all messages that belong to the same conversation.
Outlook derives it partly from the conversation topic, which is based on the subject line.
Because the subject feeds into it, the thread ID can change. When the subject changes, Outlook calculates a new thread ID.

Think of the thread ID as the label on a folder that holds related emails together. Change the subject, and Outlook decides the message belongs in a different folder, so it creates a new one.

Why the difference matters here

Tekst groups messages into threads using the thread ID, because that is the identifier Outlook itself uses to present a conversation. The Internet Message ID cannot be used for grouping, since it is unique to each individual message and never shared across a conversation.

So when a subject change forces Outlook to issue a new thread ID, Tekst has no reliable signal that the renamed messages belong to the earlier conversation. The renamed thread is genuinely new from the integration's point of view, which is why it runs through automation again.

Microsoft's recommendation

Microsoft recommends that users do not change the subject line of an ongoing email conversation. In fact, newer versions of Outlook (desktop and web) have removed the ability to edit the subject line of a reply for exactly this reason. Microsoft recognizes that changing the subject breaks the conversation thread and leads to confusion.

Tekst aligns with this recommendation. Keeping the original subject line intact ensures that all messages in a conversation are grouped correctly in both Outlook and Tekst.

What to do if this has already happened

If a subject change has already split a conversation, the threads cannot be merged back together in Tekst. You can still view both threads individually in your message feed by searching for the sender or keywords from the conversation. If the renamed thread triggered an unwanted automated action, review the affected case and correct it as you normally would.

To prevent this from happening in the future:

Do not modify the subject line when replying to or forwarding an email within an existing conversation.
Start a new email if you genuinely need a different subject - this makes the intent clear and avoids confusion.
Update to a newer version of Outlook if your organization is still using an older version that allows subject editing on replies.