fix: reply to own thread message addresses original recipients #703

lib/reply-recipients.ts

@@ -0,0 +1,108 @@
+export interface ReplyAddress {
+  email?: string;
+  name?: string;
+}
+
+export interface ReplySource {
+  from?: ReplyAddress[];
+  /** Addresses from the original message's Reply-To header. */
+  replyToAddresses?: ReplyAddress[];
+  to?: ReplyAddress[];
+  cc?: ReplyAddress[];
+}
+
+export interface ReplyRecipientsResult {
+  to: ReplyAddress[];
+  cc: ReplyAddress[];
+}
+
+function normalize(email: string): string {
+  return email.trim().toLowerCase();
+}
+
+function normalizeBase(email: string): string {
+  const normalized = normalize(email);
+  const at = normalized.indexOf('@');
+  if (at <= 0) return normalized;
+
+  const local = normalized.slice(0, at);
+  const domain = normalized.slice(at + 1);
+  const plus = local.indexOf('+');
+
+  return `${plus >= 0 ? local.slice(0, plus) : local}@${domain}`;
+}
+
+/**
+ * Does `email` belong to the user? Matches exactly first, then with `+tag`
+ * sub-addressing stripped (info+news@ is still info@).
+ */
+function isOwnAddress(email: string | undefined, ownEmails: string[]): boolean {
+  if (!email?.trim()) return false;
+  const exact = normalize(email);
+  if (ownEmails.some((own) => normalize(own) === exact)) return true;
+  const base = normalizeBase(email);
+  return ownEmails.some((own) => normalizeBase(own) === base);
+}
+
+/**
+ * Is this a message the user themself sent? True when the From address is one
+ * of their own identities - the case that shows up when browsing a thread and
+ * replying to your own last message.
+ */
+export function isSelfSent(source: ReplySource | undefined, ownEmails: string[]): boolean {
+  return isOwnAddress(source?.from?.[0]?.email, ownEmails);
+}
+
+/**
+ * Work out the To/Cc a reply should open with.
+ *
+ * Normal case: reply goes to the Reply-To header if the original carried one,
+ * else to From (RFC 5322). Reply-all adds the other original recipients,
+ * minus the user's own addresses.
+ *
+ * Self-sent case (#703): replying to your own message inside a thread must
+ * continue the conversation, not mail yourself. Gmail and Thunderbird address
+ * the reply to the message's original recipients instead, so that's what we do
+ * - the original To for reply, plus the original Cc for reply-all. Those
+ * addresses were the user's own choice, so they're kept verbatim (no self-
+ * filtering) and the Reply-To header is ignored, since answering your own
+ * Reply-To would land the mail back in your inbox again.
+ *
+ * A self-sent message with no visible recipients (Bcc-only) has nothing to
+ * continue to, so it falls back to the normal behaviour.
+ */
+export function buildReplyRecipients(
+  source: ReplySource | undefined,
+  mode: 'reply' | 'replyAll',
+  ownEmails: string[],
+): ReplyRecipientsResult {
+  if (!source) return { to: [], cc: [] };
+
+  const withEmail = (list: ReplyAddress[] | undefined) => (list ?? []).filter((r) => Boolean(r.email));
+
+  if (isSelfSent(source, ownEmails)) {
+    const originalTo = withEmail(source.to);
+    if (originalTo.length > 0) {
+      return {
+        to: originalTo,
+        cc: mode === 'replyAll' ? withEmail(source.cc) : [],
+      };
+    }
+  }
+
+  const replyTarget = withEmail(source.replyToAddresses).length
+    ? withEmail(source.replyToAddresses)
+    : (source.from?.[0]?.email ? [source.from[0]] : []);
+
+  if (mode === 'reply') {
+    return { to: replyTarget, cc: [] };
+  }
+
+  const others = (list: ReplyAddress[] | undefined) =>
+    withEmail(list).filter((r) => !isOwnAddress(r.email, ownEmails));
+
+  return {
+    to: [...replyTarget, ...others(source.to)],
+    cc: others(source.cc),
+  };
+}