Incoming email replies do not share the same topic

Situation

We have this dedicated category for incoming email on here@example.org. When someone sends an email from this address, replies are captured into this category.

Subsequent replies however, do not seem to detect they belong to the same email thread, and create a new topic with a single post every time.

What should happen?

Discourse should detect an existing topic and create a response instead of a new topic.

How to solve it?

I don’t know yet how to solve it, but I’d be interested in helping debugging this issue since we use email-in quite extensively and this is making our life difficult, especially when multiple people are involved in a conversation, as they might miss previous teammate replies and overburden third parties with potentially contradictory or redundant information.