feat: add typed message tags

README.md

@@ -101,14 +101,21 @@ SendEmailResponse response = email
 
     // Metadata and tags
     .metadata(metadata)
-    .tag("welcome", "onboarding")
+    .tag("legacy-tag")
+    .tags(
+        new MessageTag("campaign", "welcome"),
+        new MessageTag("customer", "new")
+    )
 
     // Idempotency
     .idempotencyKey("unique-request-key")
 
     .send();
 ```
 
+`tag()` remains available for the legacy single tag. The previous map form of
+`tags()` also remains available.
+
 Existing constructor-based sending usage still works:
 
 ```java

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

@@ -3,6 +3,7 @@
 import co.lettermint.client.LettermintClient;
 import co.lettermint.models.Attachment;
 import co.lettermint.models.SendEmailResponse;
+import co.lettermint.models.MessageTag;
 import co.lettermint.models.api.SendMailRequest;
 import co.lettermint.models.api.SendMailResponse;
 import com.fasterxml.jackson.core.type.TypeReference;
@@ -28,7 +29,7 @@
     private String route;
     private Map<String, Object> metadata;
     private String tag;
-    private List<Map<String, String>> tags;
+    private List<MessageTag> tags;
     private Map<String, Object> settings;
     private String idempotencyKey;
 
@@ -64,7 +65,7 @@
      * Set the sender email address.
      * Format: {@code "Name <email>"} or just {@code "email"}
      */
     public EmailEndpoint from(String from) {
         this.from = from;
         return this;
     }
@@ -72,7 +73,7 @@
     /**
      * Set recipient email addresses. Replaces any existing recipients.
      */
     public EmailEndpoint to(String... emails) {
         this.to = new ArrayList<>(Arrays.asList(emails));
         return this;
     }
@@ -80,7 +81,7 @@
     /**
      * Set CC email addresses. Replaces any existing CC recipients.
      */
     public EmailEndpoint cc(String... emails) {
         this.cc = new ArrayList<>(Arrays.asList(emails));
         return this;
     }
@@ -88,7 +89,7 @@
     /**
      * Set BCC email addresses. Replaces any existing BCC recipients.
      */
     public EmailEndpoint bcc(String... emails) {
         this.bcc = new ArrayList<>(Arrays.asList(emails));
         return this;
     }
@@ -96,7 +97,7 @@
     /**
      * Set reply-to email addresses. Replaces any existing reply-to addresses.
      */
     public EmailEndpoint replyTo(String... emails) {
         this.replyTo = new ArrayList<>(Arrays.asList(emails));
         return this;
     }
@@ -211,6 +212,9 @@
      * Set the email tag.
      */
     public EmailEndpoint tag(String tag) {
+        if (tag != null && tags.size() >= 20) {
+            throw new IllegalArgumentException("A legacy tag and no more than 19 message tags are permitted");
+        }
         this.tag = tag;
         return this;
     }
@@ -220,6 +224,27 @@
      */
     @SafeVarargs
     public final EmailEndpoint tags(Map<String, String>... tags) {
+        MessageTag[] normalized = new MessageTag[tags.length];
+        for (int index = 0; index < tags.length; index++) {
+            normalized[index] = new MessageTag(tags[index].get("name"), tags[index].get("value"));
+        }
+        return tags(normalized);
+    }
+
+    /**
+     * Set typed reusable name-value tags for the email.
+     */
+    public final EmailEndpoint tags(MessageTag... tags) {
+        int maximum = tag == null ? 20 : 19;
+        if (tags.length > maximum) {
+            throw new IllegalArgumentException("No more than " + maximum + " message tags are permitted");
+        }
+        Set<String> names = new HashSet<>();
+        for (MessageTag messageTag : tags) {
+            if (!names.add(messageTag.getName())) {
+                throw new IllegalArgumentException("Message tag names must be unique and case-sensitive");
+            }
+        }
         this.tags = new ArrayList<>(Arrays.asList(tags));
         return this;
     }

src/main/java/co/lettermint/models/MessageTag.java

@@ -0,0 +1,35 @@
+package co.lettermint.models;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+
+import java.util.Locale;
+import java.util.regex.Pattern;
+
+/** A reusable exact-match message tag. */
+public final class MessageTag {
+    private static final Pattern NAME = Pattern.compile("^[A-Za-z0-9_-]{1,32}$");
+    private static final Pattern VALUE = Pattern.compile("^[A-Za-z0-9_-]{1,64}$");
+
+    @JsonProperty("name")
+    private final String name;
+
+    @JsonProperty("value")
+    private final String value;
+
+    public MessageTag(String name, String value) {
+        if (name == null || !NAME.matcher(name).matches()) {
+            throw new IllegalArgumentException("Message tag names must match ^[A-Za-z0-9_-]{1,32}$");
+        }
+        if (name.toLowerCase(Locale.ROOT).startsWith("__lettermint")) {
+            throw new IllegalArgumentException("Message tag names must not start with __lettermint");
+        }
+        if (value == null || !VALUE.matcher(value).matches()) {
+            throw new IllegalArgumentException("Message tag values must match ^[A-Za-z0-9_-]{1,64}$");
+        }
+        this.name = name;
+        this.value = value;
+    }
+
+    public String getName() { return name; }
+    public String getValue() { return value; }
+}

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

@@ -1,6 +1,7 @@
 package co.lettermint;
 
 import co.lettermint.models.SendEmailResponse;
+import co.lettermint.models.MessageTag;
 import okhttp3.mockwebserver.MockResponse;
 import okhttp3.mockwebserver.MockWebServer;
 import okhttp3.mockwebserver.RecordedRequest;
@@ -263,4 +264,36 @@ void testSingleMetadataMethod() throws Exception {
         assertTrue(body.contains("\"key1\":\"value1\""));
         assertTrue(body.contains("\"key2\":123"));
     }
+
+    @Test
+    void testTypedMessageTags() 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")
+                .tags(new MessageTag("campaign", "welcome"), new MessageTag("customer", "new"))
+                .send();
+
+        String body = mockWebServer.takeRequest().getBody().readUtf8();
+        assertTrue(body.contains("\"tags\":[{\"name\":\"campaign\",\"value\":\"welcome\"}"));
+    }
+
+    @Test
+    void testMessageTagValidation() {
+        assertThrows(IllegalArgumentException.class, () -> new MessageTag("invalid name", "value"));
+        assertThrows(IllegalArgumentException.class, () -> new MessageTag("__LETTERMINT_internal", "value"));
+        assertThrows(IllegalArgumentException.class, () -> lettermint.email().tags(
+                new MessageTag("same", "one"), new MessageTag("same", "two")));
+
+        MessageTag[] tags = new MessageTag[20];
+        for (int index = 0; index < tags.length; index++) {
+            tags[index] = new MessageTag("tag_" + index, "value");
+        }
+        assertThrows(IllegalArgumentException.class, () -> lettermint.email().tag("legacy").tags(tags));
+    }
 }