Hi all — I’m running into a persistent threading issue in Thunderbird (v.148) vthat survives profile rebuilds and affects only specific conversations. Symptoms Some threads show the expand/collapse carrot normally. Other threads (same accounts, same folders, same time period) do not show the carrot. The pattern is consistent: the same conversations fail before and after a full profile rebuild. Both Inbox and Sent show a mix of threaded and unthreaded conversations. No global setting seems to be wrong, because most threads work correctly. What I’ve already checked mail.strict_threading = false mail.thread_without_re = true View → Sort By → Threaded View → Threads → All Folder Repair on Inbox and Sent Full profile rebuild Re‑added all accounts Rebuilt .msf files Verified that threading works across folders (Inbox ↔ Sent) Verified that threading works across accounts Verified that threading works for most conversations Header checks For the conversations that do thread, headers look normal. For the conversations that don’t thread, the headers also appear normal: Message-ID: present In-Reply-To: present References: present No obvious corruption, malformed IDs, or missing brackets. What I’m trying to understand Why would Thunderbird: Successfully thread most conversations Fail to thread specific conversations Preserve the same failures before and after a rebuild Show carrots in Inbox but not in Sent for the same conversation Show no global misconfiguration Is there a known edge case where Thunderbird rejects a thread even when the headers appear correct? Are there hidden rules about ambiguous References: chains, multi‑server storage, or cross‑account BCC workflows that could cause this? Any insight into how Thunderbird decides not to show a carrot would be appreciated. Thanks in advance.