Conversations are threaded by the "In-Reply-To" header (which is standard for modern email sending clients) or "References" header on emails. If the email does not include either, then Re:amaze will fall back to using the "Thread-Topic" header and subject line matching.
Both "In-Reply-To" and "References" headers reference Message-IDs, which are unique email identifiers. Re:amaze uses these Message-IDs to properly find and thread conversations accordingly.
The "Thread-Topic" header does not include unique email identifiers, therefore we will look for a matching subject line in Re:amaze. Archived and spam conversations are excluded in this type of matching. However, if the incoming email has a matching reply prefix (i.e. "Re:"), Re:amaze will re-open and thread the response in the corresponding archived conversation.
The list of reply prefixes we honor are listed here: https://en.wikipedia.org/wiki/List_of_email_subject_abbreviations#Abbreviations_in_other_languages