Conversations

The mail list shows conversations: a message and the replies to it, from every folder, in one row. Viauna works them out as it indexes each message (server/mail/threading.ts), from the headers mail apps put on replies, not by guessing.

What joins a conversation

• A reply. Every message has a Message-ID; a reply names the one it answers in In-Reply-To, and the conversation so far in References. A message whose In-Reply-To or References names one already in a conversation joins it.
• Across folders. Your reply, filed in Sent, sits under the message it answers in the Inbox.
• In any order. A reply that arrives before the message it answers (from a slow server, or synced first) keeps the ids it names, so the original joins it when it comes.
• Two branches. When a later message names messages from two conversations, they become one.
• A copy. The same message in another folder (the same Message-ID from the same sender), such as a copy filtered elsewhere, is in the same conversation, whatever its subject.

What starts a new one

• A new subject. A reply whose subject differs from the conversation's starts a new conversation, rather than being glued onto an old one. Subjects are compared without Re:, Fwd: and their equivalents in other languages (AW:, SV:, Antw:, RIF:, Odp: and more, with counts such as Re[2]:), without mailing-list tags such as [team], and ignoring case and spacing.
• No reply headers. A message that names no other, as some automatic senders send, is a conversation of its own even with the same subject as another: nothing shows it answers that one.
• A stranger's copy. A message reusing the Message-ID of one of yours, from another sender, doesn't join or take over your conversation.

Where it's kept

The conversations are part of the mail index (index.db), worked out again if it's rebuilt. Nothing is written to your mail server: other mail apps group the same mail in their own way.