diff --git a/internal/cmd/mail.go b/internal/cmd/mail.go
new file mode 100644
index 00000000..1fc6568d
--- /dev/null
+++ b/internal/cmd/mail.go
@@ -0,0 +1,285 @@
+package cmd
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/spf13/cobra"
+	"github.com/steveyegge/gastown/internal/mail"
+	"github.com/steveyegge/gastown/internal/style"
+	"github.com/steveyegge/gastown/internal/workspace"
+)
+
+// Mail command flags
+var (
+	mailSubject    string
+	mailBody       string
+	mailPriority   string
+	mailNotify     bool
+	mailInboxJSON  bool
+	mailReadJSON   bool
+	mailInboxUnread bool
+)
+
+var mailCmd = &cobra.Command{
+	Use:   "mail",
+	Short: "Agent messaging system",
+	Long: `Send and receive messages between agents.
+
+The mail system allows Mayor, polecats, and the Refinery to communicate.`,
+}
+
+var mailSendCmd = &cobra.Command{
+	Use:   "send <address>",
+	Short: "Send a message",
+	Long: `Send a message to an agent.
+
+Addresses:
+  mayor/           - Send to Mayor
+  <rig>/refinery   - Send to a rig's Refinery
+  <rig>/<polecat>  - Send to a specific polecat
+  <rig>/           - Broadcast to a rig
+
+Examples:
+  gt mail send gastown/Toast -s "Status check" -m "How's that bug fix going?"
+  gt mail send mayor/ -s "Work complete" -m "Finished gt-abc"
+  gt mail send gastown/ -s "All hands" -m "Swarm starting" --notify`,
+	Args: cobra.ExactArgs(1),
+	RunE: runMailSend,
+}
+
+var mailInboxCmd = &cobra.Command{
+	Use:   "inbox [address]",
+	Short: "Check inbox",
+	Long: `Check messages in an inbox.
+
+If no address is specified, shows the current context's inbox.
+
+Examples:
+  gt mail inbox             # Current context
+  gt mail inbox mayor/      # Mayor's inbox
+  gt mail inbox gastown/Toast  # Polecat's inbox`,
+	Args: cobra.MaximumNArgs(1),
+	RunE: runMailInbox,
+}
+
+var mailReadCmd = &cobra.Command{
+	Use:   "read <message-id>",
+	Short: "Read a message",
+	Long: `Read a specific message and mark it as read.
+
+The message ID can be found from 'gt mail inbox'.`,
+	Args: cobra.ExactArgs(1),
+	RunE: runMailRead,
+}
+
+func init() {
+	// Send flags
+	mailSendCmd.Flags().StringVarP(&mailSubject, "subject", "s", "", "Message subject (required)")
+	mailSendCmd.Flags().StringVarP(&mailBody, "message", "m", "", "Message body")
+	mailSendCmd.Flags().StringVar(&mailPriority, "priority", "normal", "Message priority (normal, high)")
+	mailSendCmd.Flags().BoolVarP(&mailNotify, "notify", "n", false, "Send tmux notification to recipient")
+	mailSendCmd.MarkFlagRequired("subject")
+
+	// Inbox flags
+	mailInboxCmd.Flags().BoolVar(&mailInboxJSON, "json", false, "Output as JSON")
+	mailInboxCmd.Flags().BoolVarP(&mailInboxUnread, "unread", "u", false, "Show only unread messages")
+
+	// Read flags
+	mailReadCmd.Flags().BoolVar(&mailReadJSON, "json", false, "Output as JSON")
+
+	// Add subcommands
+	mailCmd.AddCommand(mailSendCmd)
+	mailCmd.AddCommand(mailInboxCmd)
+	mailCmd.AddCommand(mailReadCmd)
+
+	rootCmd.AddCommand(mailCmd)
+}
+
+func runMailSend(cmd *cobra.Command, args []string) error {
+	to := args[0]
+
+	townRoot, err := workspace.FindFromCwdOrError()
+	if err != nil {
+		return fmt.Errorf("not in a Gas Town workspace: %w", err)
+	}
+
+	// Determine sender
+	from := detectSender(townRoot)
+
+	// Create message
+	msg := mail.NewMessage(from, to, mailSubject, mailBody)
+
+	// Set priority
+	if mailPriority == "high" || mailNotify {
+		msg.Priority = mail.PriorityHigh
+	}
+
+	// Send
+	router := mail.NewRouter(townRoot)
+	if err := router.Send(msg); err != nil {
+		return fmt.Errorf("sending message: %w", err)
+	}
+
+	fmt.Printf("%s Message sent to %s\n", style.Bold.Render("✓"), to)
+	fmt.Printf("  ID: %s\n", style.Dim.Render(msg.ID))
+	fmt.Printf("  Subject: %s\n", mailSubject)
+
+	return nil
+}
+
+func runMailInbox(cmd *cobra.Command, args []string) error {
+	townRoot, err := workspace.FindFromCwdOrError()
+	if err != nil {
+		return fmt.Errorf("not in a Gas Town workspace: %w", err)
+	}
+
+	// Determine which inbox to check
+	address := ""
+	if len(args) > 0 {
+		address = args[0]
+	} else {
+		address = detectSender(townRoot)
+	}
+
+	// Get mailbox
+	router := mail.NewRouter(townRoot)
+	mailbox, err := router.GetMailbox(address)
+	if err != nil {
+		return fmt.Errorf("getting mailbox: %w", err)
+	}
+
+	// Get messages
+	var messages []*mail.Message
+	if mailInboxUnread {
+		messages, err = mailbox.ListUnread()
+	} else {
+		messages, err = mailbox.List()
+	}
+	if err != nil {
+		return fmt.Errorf("listing messages: %w", err)
+	}
+
+	// JSON output
+	if mailInboxJSON {
+		enc := json.NewEncoder(os.Stdout)
+		enc.SetIndent("", "  ")
+		return enc.Encode(messages)
+	}
+
+	// Human-readable output
+	total, unread, _ := mailbox.Count()
+	fmt.Printf("%s Inbox: %s (%d messages, %d unread)\n\n",
+		style.Bold.Render("📬"), address, total, unread)
+
+	if len(messages) == 0 {
+		fmt.Printf("  %s\n", style.Dim.Render("(no messages)"))
+		return nil
+	}
+
+	for _, msg := range messages {
+		readMarker := "●"
+		if msg.Read {
+			readMarker = "○"
+		}
+		priorityMarker := ""
+		if msg.Priority == mail.PriorityHigh {
+			priorityMarker = " " + style.Bold.Render("!")
+		}
+
+		fmt.Printf("  %s %s%s\n", readMarker, msg.Subject, priorityMarker)
+		fmt.Printf("    %s from %s\n",
+			style.Dim.Render(msg.ID),
+			msg.From)
+		fmt.Printf("    %s\n",
+			style.Dim.Render(msg.Timestamp.Format("2006-01-02 15:04")))
+	}
+
+	return nil
+}
+
+func runMailRead(cmd *cobra.Command, args []string) error {
+	msgID := args[0]
+
+	townRoot, err := workspace.FindFromCwdOrError()
+	if err != nil {
+		return fmt.Errorf("not in a Gas Town workspace: %w", err)
+	}
+
+	// Determine which inbox
+	address := detectSender(townRoot)
+
+	// Get mailbox and message
+	router := mail.NewRouter(townRoot)
+	mailbox, err := router.GetMailbox(address)
+	if err != nil {
+		return fmt.Errorf("getting mailbox: %w", err)
+	}
+
+	msg, err := mailbox.Get(msgID)
+	if err != nil {
+		return fmt.Errorf("getting message: %w", err)
+	}
+
+	// Mark as read
+	mailbox.MarkRead(msgID)
+
+	// JSON output
+	if mailReadJSON {
+		enc := json.NewEncoder(os.Stdout)
+		enc.SetIndent("", "  ")
+		return enc.Encode(msg)
+	}
+
+	// Human-readable output
+	priorityStr := ""
+	if msg.Priority == mail.PriorityHigh {
+		priorityStr = " " + style.Bold.Render("[HIGH PRIORITY]")
+	}
+
+	fmt.Printf("%s %s%s\n\n", style.Bold.Render("Subject:"), msg.Subject, priorityStr)
+	fmt.Printf("From: %s\n", msg.From)
+	fmt.Printf("To: %s\n", msg.To)
+	fmt.Printf("Date: %s\n", msg.Timestamp.Format("2006-01-02 15:04:05"))
+	fmt.Printf("ID: %s\n", style.Dim.Render(msg.ID))
+
+	if msg.Body != "" {
+		fmt.Printf("\n%s\n", msg.Body)
+	}
+
+	return nil
+}
+
+// detectSender determines the current context's address.
+func detectSender(townRoot string) string {
+	// Check environment variables (set by session start)
+	rig := os.Getenv("GT_RIG")
+	polecat := os.Getenv("GT_POLECAT")
+
+	if rig != "" && polecat != "" {
+		return fmt.Sprintf("%s/%s", rig, polecat)
+	}
+
+	// Check current directory
+	cwd, err := os.Getwd()
+	if err != nil {
+		return "mayor/"
+	}
+
+	// If in a rig's polecats directory, extract address
+	if strings.Contains(cwd, "/polecats/") {
+		parts := strings.Split(cwd, "/polecats/")
+		if len(parts) >= 2 {
+			rigPath := parts[0]
+			polecatPath := strings.Split(parts[1], "/")[0]
+			rigName := filepath.Base(rigPath)
+			return fmt.Sprintf("%s/%s", rigName, polecatPath)
+		}
+	}
+
+	// Default to mayor
+	return "mayor/"
+}
diff --git a/internal/mail/mailbox.go b/internal/mail/mailbox.go
new file mode 100644
index 00000000..ec4f91ac
--- /dev/null
+++ b/internal/mail/mailbox.go
@@ -0,0 +1,222 @@
+package mail
+
+import (
+	"bufio"
+	"encoding/json"
+	"errors"
+	"os"
+	"path/filepath"
+	"sort"
+)
+
+// Common errors
+var (
+	ErrMessageNotFound = errors.New("message not found")
+	ErrEmptyInbox      = errors.New("inbox is empty")
+)
+
+// Mailbox manages a JSONL-based inbox.
+type Mailbox struct {
+	path string
+}
+
+// NewMailbox creates a mailbox at the given path.
+func NewMailbox(path string) *Mailbox {
+	return &Mailbox{path: path}
+}
+
+// Path returns the mailbox file path.
+func (m *Mailbox) Path() string {
+	return m.path
+}
+
+// List returns all messages in the mailbox.
+func (m *Mailbox) List() ([]*Message, error) {
+	file, err := os.Open(m.path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, nil
+		}
+		return nil, err
+	}
+	defer file.Close()
+
+	var messages []*Message
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		line := scanner.Text()
+		if line == "" {
+			continue
+		}
+
+		var msg Message
+		if err := json.Unmarshal([]byte(line), &msg); err != nil {
+			continue // Skip malformed lines
+		}
+		messages = append(messages, &msg)
+	}
+
+	if err := scanner.Err(); err != nil {
+		return nil, err
+	}
+
+	// Sort by timestamp (newest first)
+	sort.Slice(messages, func(i, j int) bool {
+		return messages[i].Timestamp.After(messages[j].Timestamp)
+	})
+
+	return messages, nil
+}
+
+// ListUnread returns unread messages.
+func (m *Mailbox) ListUnread() ([]*Message, error) {
+	all, err := m.List()
+	if err != nil {
+		return nil, err
+	}
+
+	var unread []*Message
+	for _, msg := range all {
+		if !msg.Read {
+			unread = append(unread, msg)
+		}
+	}
+
+	return unread, nil
+}
+
+// Get returns a message by ID.
+func (m *Mailbox) Get(id string) (*Message, error) {
+	messages, err := m.List()
+	if err != nil {
+		return nil, err
+	}
+
+	for _, msg := range messages {
+		if msg.ID == id {
+			return msg, nil
+		}
+	}
+
+	return nil, ErrMessageNotFound
+}
+
+// Append adds a message to the mailbox.
+func (m *Mailbox) Append(msg *Message) error {
+	// Ensure directory exists
+	dir := filepath.Dir(m.path)
+	if err := os.MkdirAll(dir, 0755); err != nil {
+		return err
+	}
+
+	// Open for append
+	file, err := os.OpenFile(m.path, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
+	if err != nil {
+		return err
+	}
+	defer file.Close()
+
+	data, err := json.Marshal(msg)
+	if err != nil {
+		return err
+	}
+
+	_, err = file.WriteString(string(data) + "\n")
+	return err
+}
+
+// MarkRead marks a message as read.
+func (m *Mailbox) MarkRead(id string) error {
+	messages, err := m.List()
+	if err != nil {
+		return err
+	}
+
+	found := false
+	for _, msg := range messages {
+		if msg.ID == id {
+			msg.Read = true
+			found = true
+		}
+	}
+
+	if !found {
+		return ErrMessageNotFound
+	}
+
+	return m.rewrite(messages)
+}
+
+// Delete removes a message from the mailbox.
+func (m *Mailbox) Delete(id string) error {
+	messages, err := m.List()
+	if err != nil {
+		return err
+	}
+
+	var filtered []*Message
+	found := false
+	for _, msg := range messages {
+		if msg.ID == id {
+			found = true
+		} else {
+			filtered = append(filtered, msg)
+		}
+	}
+
+	if !found {
+		return ErrMessageNotFound
+	}
+
+	return m.rewrite(filtered)
+}
+
+// Count returns the total and unread message counts.
+func (m *Mailbox) Count() (total, unread int, err error) {
+	messages, err := m.List()
+	if err != nil {
+		return 0, 0, err
+	}
+
+	total = len(messages)
+	for _, msg := range messages {
+		if !msg.Read {
+			unread++
+		}
+	}
+
+	return total, unread, nil
+}
+
+// rewrite rewrites the mailbox with the given messages.
+func (m *Mailbox) rewrite(messages []*Message) error {
+	// Sort by timestamp (oldest first for JSONL)
+	sort.Slice(messages, func(i, j int) bool {
+		return messages[i].Timestamp.Before(messages[j].Timestamp)
+	})
+
+	// Write to temp file
+	tmpPath := m.path + ".tmp"
+	file, err := os.Create(tmpPath)
+	if err != nil {
+		return err
+	}
+
+	for _, msg := range messages {
+		data, err := json.Marshal(msg)
+		if err != nil {
+			file.Close()
+			os.Remove(tmpPath)
+			return err
+		}
+		file.WriteString(string(data) + "\n")
+	}
+
+	if err := file.Close(); err != nil {
+		os.Remove(tmpPath)
+		return err
+	}
+
+	// Atomic rename
+	return os.Rename(tmpPath, m.path)
+}
diff --git a/internal/mail/router.go b/internal/mail/router.go
new file mode 100644
index 00000000..b6c9ccf8
--- /dev/null
+++ b/internal/mail/router.go
@@ -0,0 +1,136 @@
+package mail
+
+import (
+	"fmt"
+	"path/filepath"
+	"strings"
+
+	"github.com/steveyegge/gastown/internal/tmux"
+)
+
+// Router handles message delivery and address resolution.
+type Router struct {
+	townRoot string
+	tmux     *tmux.Tmux
+}
+
+// NewRouter creates a new mail router.
+func NewRouter(townRoot string) *Router {
+	return &Router{
+		townRoot: townRoot,
+		tmux:     tmux.NewTmux(),
+	}
+}
+
+// Send delivers a message to its recipient.
+func (r *Router) Send(msg *Message) error {
+	// Resolve recipient mailbox path
+	mailboxPath, err := r.ResolveMailbox(msg.To)
+	if err != nil {
+		return fmt.Errorf("resolving address '%s': %w", msg.To, err)
+	}
+
+	// Append to mailbox
+	mailbox := NewMailbox(mailboxPath)
+	if err := mailbox.Append(msg); err != nil {
+		return fmt.Errorf("delivering message: %w", err)
+	}
+
+	// Optionally notify if recipient is a polecat with active session
+	if isPolecat(msg.To) && msg.Priority == PriorityHigh {
+		r.notifyPolecat(msg)
+	}
+
+	return nil
+}
+
+// ResolveMailbox converts an address to a mailbox file path.
+//
+// Address formats:
+//   - mayor/           → <town>/mayor/mail/inbox.jsonl
+//   - <rig>/refinery   → <town>/<rig>/refinery/mail/inbox.jsonl
+//   - <rig>/<polecat>  → <town>/<rig>/polecats/<polecat>/mail/inbox.jsonl
+//   - <rig>/           → <town>/<rig>/mail/inbox.jsonl (rig broadcast)
+func (r *Router) ResolveMailbox(address string) (string, error) {
+	address = strings.TrimSpace(address)
+	if address == "" {
+		return "", fmt.Errorf("empty address")
+	}
+
+	// Mayor
+	if address == "mayor/" || address == "mayor" {
+		return filepath.Join(r.townRoot, "mayor", "mail", "inbox.jsonl"), nil
+	}
+
+	// Parse rig/target
+	parts := strings.SplitN(address, "/", 2)
+	if len(parts) < 2 {
+		return "", fmt.Errorf("invalid address format: %s", address)
+	}
+
+	rig := parts[0]
+	target := parts[1]
+
+	// Rig broadcast (empty target or just /)
+	if target == "" {
+		return filepath.Join(r.townRoot, rig, "mail", "inbox.jsonl"), nil
+	}
+
+	// Refinery
+	if target == "refinery" {
+		return filepath.Join(r.townRoot, rig, "refinery", "mail", "inbox.jsonl"), nil
+	}
+
+	// Polecat
+	return filepath.Join(r.townRoot, rig, "polecats", target, "mail", "inbox.jsonl"), nil
+}
+
+// GetMailbox returns a Mailbox for the given address.
+func (r *Router) GetMailbox(address string) (*Mailbox, error) {
+	path, err := r.ResolveMailbox(address)
+	if err != nil {
+		return nil, err
+	}
+	return NewMailbox(path), nil
+}
+
+// notifyPolecat sends a notification to a polecat's tmux session.
+func (r *Router) notifyPolecat(msg *Message) error {
+	// Parse rig/polecat from address
+	parts := strings.SplitN(msg.To, "/", 2)
+	if len(parts) != 2 {
+		return nil
+	}
+
+	rig := parts[0]
+	polecat := parts[1]
+
+	// Generate session name (matches session.Manager)
+	sessionID := fmt.Sprintf("gt-%s-%s", rig, polecat)
+
+	// Check if session exists
+	hasSession, err := r.tmux.HasSession(sessionID)
+	if err != nil || !hasSession {
+		return nil // No active session, skip notification
+	}
+
+	// Inject notification
+	notification := fmt.Sprintf("[MAIL] %s", msg.Subject)
+	return r.tmux.SendKeys(sessionID, notification)
+}
+
+// isPolecat checks if an address points to a polecat.
+func isPolecat(address string) bool {
+	// Not mayor, not refinery, has rig/name format
+	if strings.HasPrefix(address, "mayor") {
+		return false
+	}
+
+	parts := strings.SplitN(address, "/", 2)
+	if len(parts) != 2 {
+		return false
+	}
+
+	target := parts[1]
+	return target != "" && target != "refinery"
+}
diff --git a/internal/mail/types.go b/internal/mail/types.go
new file mode 100644
index 00000000..af15fa20
--- /dev/null
+++ b/internal/mail/types.go
@@ -0,0 +1,67 @@
+// Package mail provides JSONL-based messaging for agent communication.
+package mail
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"time"
+)
+
+// Priority levels for messages.
+type Priority string
+
+const (
+	// PriorityNormal is the default priority.
+	PriorityNormal Priority = "normal"
+
+	// PriorityHigh indicates an urgent message.
+	PriorityHigh Priority = "high"
+)
+
+// Message represents a mail message between agents.
+type Message struct {
+	// ID is a unique message identifier.
+	ID string `json:"id"`
+
+	// From is the sender address (e.g., "gastown/Toast" or "mayor/").
+	From string `json:"from"`
+
+	// To is the recipient address.
+	To string `json:"to"`
+
+	// Subject is a brief summary.
+	Subject string `json:"subject"`
+
+	// Body is the full message content.
+	Body string `json:"body"`
+
+	// Timestamp is when the message was sent.
+	Timestamp time.Time `json:"timestamp"`
+
+	// Read indicates if the message has been read.
+	Read bool `json:"read"`
+
+	// Priority is the message priority.
+	Priority Priority `json:"priority"`
+}
+
+// NewMessage creates a new message with a generated ID.
+func NewMessage(from, to, subject, body string) *Message {
+	return &Message{
+		ID:        generateID(),
+		From:      from,
+		To:        to,
+		Subject:   subject,
+		Body:      body,
+		Timestamp: time.Now(),
+		Read:      false,
+		Priority:  PriorityNormal,
+	}
+}
+
+// generateID creates a random message ID.
+func generateID() string {
+	b := make([]byte, 8)
+	rand.Read(b)
+	return "msg-" + hex.EncodeToString(b)
+}