fix(gmail): use Reply-To header for reply-all per RFC 5322

When the original message has a Reply-To header, use that address
instead of From when auto-populating recipients for --reply-all.
This follows RFC 5322 email conventions for reply handling.

- Add ReplyToAddr field to replyInfo struct
- Fetch Reply-To header in fetchReplyInfo()
- Update buildReplyAllRecipients to prioritize Reply-To over From
- Add tests for Reply-To header behavior

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: salmonumbrella

internal/cmd/gmail_send.go

@@ -151,15 +151,22 @@ func (c *GmailSendCmd) Run(ctx context.Context, flags *RootFlags) error {
 }
 
 // buildReplyAllRecipients constructs To and Cc lists for a reply-all.
-// Original sender (From) -> To
+// Per RFC 5322: if Reply-To header is present, use it instead of From.
+// Reply-To (or From if no Reply-To) -> To
 // Original To recipients -> To
 // Original Cc recipients -> Cc
 // Filters out self and deduplicates.
 func buildReplyAllRecipients(info *replyInfo, selfEmail string) (to, cc []string) {
-	// Collect To recipients: original sender + original To recipients
+	// Collect To recipients: reply address (Reply-To if present, else From) + original To recipients
 	toAddrs := make([]string, 0, 1+len(info.ToAddrs))
-	if fromAddrs := parseEmailAddresses(info.FromAddr); len(fromAddrs) > 0 {
-		toAddrs = append(toAddrs, fromAddrs...)
+
+	// Per RFC 5322, Reply-To takes precedence over From for replies
+	replyAddress := info.ReplyToAddr
+	if replyAddress == "" {
+		replyAddress = info.FromAddr
+	}
+	if replyAddrs := parseEmailAddresses(replyAddress); len(replyAddrs) > 0 {
+		toAddrs = append(toAddrs, replyAddrs...)
 	}
 	toAddrs = append(toAddrs, info.ToAddrs...)
 
@@ -188,12 +195,13 @@ func buildReplyAllRecipients(info *replyInfo, selfEmail string) (to, cc []string
 
 // replyInfo contains all information extracted from the original message for replying
 type replyInfo struct {
-	InReplyTo  string
-	References string
-	ThreadID   string
-	FromAddr   string   // Original sender
-	ToAddrs    []string // Original To recipients
-	CcAddrs    []string // Original Cc recipients
+	InReplyTo   string
+	References  string
+	ThreadID    string
+	FromAddr    string   // Original sender
+	ReplyToAddr string   // Original Reply-To header (per RFC 5322, use this instead of From if present)
+	ToAddrs     []string // Original To recipients
+	CcAddrs     []string // Original Cc recipients
 }
 
 func replyHeaders(ctx context.Context, svc *gmail.Service, replyToMessageID string) (inReplyTo string, references string, threadID string, err error) {
@@ -211,18 +219,19 @@ func fetchReplyInfo(ctx context.Context, svc *gmail.Service, replyToMessageID st
 	}
 	msg, err := svc.Users.Messages.Get("me", replyToMessageID).
 		Format("metadata").
-		MetadataHeaders("Message-ID", "Message-Id", "References", "In-Reply-To", "From", "To", "Cc").
+		MetadataHeaders("Message-ID", "Message-Id", "References", "In-Reply-To", "From", "Reply-To", "To", "Cc").
 		Context(ctx).
 		Do()
 	if err != nil {
 		return nil, err
 	}
 
 	info := &replyInfo{
-		ThreadID: msg.ThreadId,
-		FromAddr: headerValue(msg.Payload, "From"),
-		ToAddrs:  parseEmailAddresses(headerValue(msg.Payload, "To")),
-		CcAddrs:  parseEmailAddresses(headerValue(msg.Payload, "Cc")),
+		ThreadID:    msg.ThreadId,
+		FromAddr:    headerValue(msg.Payload, "From"),
+		ReplyToAddr: headerValue(msg.Payload, "Reply-To"),
+		ToAddrs:     parseEmailAddresses(headerValue(msg.Payload, "To")),
+		CcAddrs:     parseEmailAddresses(headerValue(msg.Payload, "Cc")),
 	}
 
 	// Prefer Message-ID and References from the original message.

internal/cmd/gmail_send_test.go

@@ -323,6 +323,42 @@ func TestBuildReplyAllRecipients(t *testing.T) {
 			expectTo:  []string{},
 			expectCc:  []string{},
 		},
+		{
+			name: "Reply-To header takes precedence over From (RFC 5322)",
+			info: &replyInfo{
+				FromAddr:    "original-sender@example.com",
+				ReplyToAddr: "reply-here@example.com",
+				ToAddrs:     []string{"me@example.com", "alice@example.com"},
+				CcAddrs:     nil,
+			},
+			selfEmail: "me@example.com",
+			expectTo:  []string{"reply-here@example.com", "alice@example.com"},
+			expectCc:  []string{},
+		},
+		{
+			name: "Reply-To with display name",
+			info: &replyInfo{
+				FromAddr:    "sender@example.com",
+				ReplyToAddr: "Mailing List <list@example.com>",
+				ToAddrs:     []string{"alice@example.com"},
+				CcAddrs:     nil,
+			},
+			selfEmail: "me@example.com",
+			expectTo:  []string{"list@example.com", "alice@example.com"},
+			expectCc:  []string{},
+		},
+		{
+			name: "Empty Reply-To falls back to From",
+			info: &replyInfo{
+				FromAddr:    "sender@example.com",
+				ReplyToAddr: "",
+				ToAddrs:     []string{"alice@example.com"},
+				CcAddrs:     nil,
+			},
+			selfEmail: "me@example.com",
+			expectTo:  []string{"sender@example.com", "alice@example.com"},
+			expectCc:  []string{},
+		},
 	}
 
 	for _, tc := range tests {
@@ -373,6 +409,15 @@ func TestFetchReplyInfo(t *testing.T) {
 				{Name: "To", Value: "recipient@example.com"},
 			},
 		},
+		"m3": {
+			ThreadID: "t3",
+			Headers: []hdr{
+				{Name: "Message-ID", Value: "<id3@example.com>"},
+				{Name: "From", Value: "original-sender@example.com"},
+				{Name: "Reply-To", Value: "Mailing List <list@example.com>"},
+				{Name: "To", Value: "recipient@example.com"},
+			},
+		},
 	}
 
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
@@ -450,6 +495,18 @@ func TestFetchReplyInfo(t *testing.T) {
 	if info.ThreadID != "" || info.FromAddr != "" {
 		t.Errorf("Expected empty replyInfo for empty message ID")
 	}
+
+	// Test m3: message with Reply-To header
+	info, err = fetchReplyInfo(ctx, svc, "m3")
+	if err != nil {
+		t.Fatalf("fetchReplyInfo(m3): %v", err)
+	}
+	if info.FromAddr != "original-sender@example.com" {
+		t.Errorf("FromAddr = %q, want %q", info.FromAddr, "original-sender@example.com")
+	}
+	if info.ReplyToAddr != "Mailing List <list@example.com>" {
+		t.Errorf("ReplyToAddr = %q, want %q", info.ReplyToAddr, "Mailing List <list@example.com>")
+	}
 }
 
 func TestReplyAllValidation(t *testing.T) {