My email replies are not threading correctly.

Threading is resolved using a chain of fallbacks, in order.

Reply headers

Most mail clients preserve the In-Reply-To and References headers automatically, which is the most reliable signal we use.

Reply-To address

Every outbound message sets a unique Reply-To address that encodes the conversation, which survives even if a client strips the other headers.

Subject matching

As a last resort we match on a normalised subject line and sender address within a 30 day window. Changing the subject line of a reply can break this fallback, so we recommend leaving it as is.