fix: use correct reply-to type

src/main/java/co/lettermint/endpoints/EmailEndpoint.java

@@ -15,7 +15,7 @@ public class EmailEndpoint extends Endpoint {
     private List<String> to;
     private List<String> cc;
     private List<String> bcc;
-    private String replyTo;
+    private List<String> replyTo;
     private String subject;
     private String html;
     private String text;
@@ -39,7 +39,7 @@ private void reset() {
         this.to = new ArrayList<>();
         this.cc = new ArrayList<>();
         this.bcc = new ArrayList<>();
-        this.replyTo = null;
+        this.replyTo = new ArrayList<>();
         this.subject = null;
         this.html = null;
         this.text = null;
@@ -85,10 +85,10 @@ public EmailEndpoint bcc(String... emails) {
     }
 
     /**
-     * Set the reply-to email address.
+     * Set reply-to email addresses. Replaces any existing reply-to addresses.
      */
-    public EmailEndpoint replyTo(String replyTo) {
-        this.replyTo = replyTo;
+    public EmailEndpoint replyTo(String... emails) {
+        this.replyTo = new ArrayList<>(Arrays.asList(emails));
         return this;
     }
 
@@ -233,7 +233,7 @@ private Map<String, Object> buildPayload() {
             payload.put("bcc", bcc);
         }
 
-        if (replyTo != null) {
+        if (!replyTo.isEmpty()) {
             payload.put("reply_to", replyTo);
         }
 

src/test/java/co/lettermint/EmailEndpointTest.java

@@ -97,7 +97,7 @@ void testEmailWithAllOptions() throws Exception {
         assertTrue(body.contains("\"to\":[\"recipient1@example.com\",\"recipient2@example.com\"]"));
         assertTrue(body.contains("\"cc\":[\"cc@example.com\"]"));
         assertTrue(body.contains("\"bcc\":[\"bcc@example.com\"]"));
-        assertTrue(body.contains("\"reply_to\":\"reply@example.com\""));
+        assertTrue(body.contains("\"reply_to\":[\"reply@example.com\"]"));
         assertTrue(body.contains("\"subject\":\"Welcome!\""));
         assertTrue(body.contains("\"html\":\"<p>Hello <b>World</b></p>\""));
         assertTrue(body.contains("\"text\":\"Hello World\""));
@@ -179,6 +179,46 @@ void testSingleHeaderMethod() throws Exception {
         assertTrue(body.contains("\"X-Second\":\"value2\""));
     }
 
+    @Test
+    void testReplyToSingleString() throws Exception {
+        mockWebServer.enqueue(new MockResponse()
+                .setBody("{\"message_id\": \"msg_123\", \"status\": \"queued\"}")
+                .setHeader("Content-Type", "application/json"));
+
+        lettermint.email()
+                .from("sender@example.com")
+                .to("recipient@example.com")
+                .subject("Test")
+                .text("Test")
+                .replyTo("reply@example.com")
+                .send();
+
+        RecordedRequest request = mockWebServer.takeRequest();
+        String body = request.getBody().readUtf8();
+
+        assertTrue(body.contains("\"reply_to\":[\"reply@example.com\"]"));
+    }
+
+    @Test
+    void testReplyToMultipleAddresses() throws Exception {
+        mockWebServer.enqueue(new MockResponse()
+                .setBody("{\"message_id\": \"msg_123\", \"status\": \"queued\"}")
+                .setHeader("Content-Type", "application/json"));
+
+        lettermint.email()
+                .from("sender@example.com")
+                .to("recipient@example.com")
+                .subject("Test")
+                .text("Test")
+                .replyTo("reply1@example.com", "reply2@example.com")
+                .send();
+
+        RecordedRequest request = mockWebServer.takeRequest();
+        String body = request.getBody().readUtf8();
+
+        assertTrue(body.contains("\"reply_to\":[\"reply1@example.com\",\"reply2@example.com\"]"));
+    }
+
     @Test
     void testSingleMetadataMethod() throws Exception {
         mockWebServer.enqueue(new MockResponse()