diff --git a/README.md b/README.md
index 8a61fa3..bd372f3 100644
--- a/README.md
+++ b/README.md
@@ -61,9 +61,16 @@ const response = await email
     foo: 'bar',
   })
   .tag('campaign-123')
+  .tags([
+    { name: 'campaign', value: 'welcome' },
+    { name: 'customer', value: 'new' },
+  ])
   .send();
 ```
 
+`tag()` remains available for the legacy single tag. `tags()` accepts up to 20
+case-sensitive name/value tags, or 19 when `tag()` is also set.
+
 The legacy constructor still works for sending-only usage:
 
 ```typescript
diff --git a/src/endpoints/email.spec.ts b/src/endpoints/email.spec.ts
index 906b55e..9e49916 100644
--- a/src/endpoints/email.spec.ts
+++ b/src/endpoints/email.spec.ts
@@ -318,6 +318,28 @@ describe('EmailEndpoint', () => {
     });
   });
 
+  it.each([
+    [
+      [
+        { name: 'duplicate', value: 'one' },
+        { name: 'duplicate', value: 'two' },
+      ],
+    ],
+    [[{ name: '__LETTERMint_internal', value: 'one' }]],
+    [[{ name: 'invalid name', value: 'one' }]],
+    [[{ name: 'valid', value: 'invalid value' }]],
+  ])('should reject invalid reusable tags', (tags) => {
+    expect(() => emailEndpoint.tags(tags)).toThrow(TypeError);
+  });
+
+  it('should count the legacy tag in the message tag limit', () => {
+    const tags = Array.from({ length: 20 }, (_, index) => ({
+      name: `tag_${index}`,
+      value: 'value',
+    }));
+    expect(() => emailEndpoint.tag('legacy').tags(tags)).toThrow(TypeError);
+  });
+
   it('should send the email with all options', async () => {
     // Set up a complete email
     emailEndpoint
diff --git a/src/endpoints/email.ts b/src/endpoints/email.ts
index fbb489b..02e0d6b 100644
--- a/src/endpoints/email.ts
+++ b/src/endpoints/email.ts
@@ -237,6 +237,9 @@ export class EmailEndpoint extends Endpoint {
    * @returns The current instance for chaining
    */
   public tag(tag: string): this {
+    if ((this.payload.tags?.length ?? 0) >= 20) {
+      throw new TypeError('A legacy tag and no more than 19 message tags are permitted');
+    }
     this.payload.tag = tag;
     return this;
   }
@@ -248,6 +251,30 @@ export class EmailEndpoint extends Endpoint {
    * @returns The current instance for chaining
    */
   public tags(tags: NonNullable<EmailPayload['tags']>): this {
+    const maximum = this.payload.tag == null ? 20 : 19;
+    if (tags.length > maximum) {
+      throw new TypeError(
+        `No more than ${maximum} message tags are permitted with the current legacy tag`
+      );
+    }
+
+    const names = new Set<string>();
+    for (const tag of tags) {
+      if (tag.name.length < 1 || tag.name.length > 32 || !/^[A-Za-z0-9_-]+$/.test(tag.name)) {
+        throw new TypeError('Message tag names must match ^[A-Za-z0-9_-]{1,32}$');
+      }
+      if (tag.name.toLowerCase().startsWith('__lettermint')) {
+        throw new TypeError('Message tag names must not start with __lettermint');
+      }
+      if (tag.value.length < 1 || tag.value.length > 64 || !/^[A-Za-z0-9_-]+$/.test(tag.value)) {
+        throw new TypeError('Message tag values must match ^[A-Za-z0-9_-]{1,64}$');
+      }
+      if (names.has(tag.name)) {
+        throw new TypeError(`Duplicate message tag name: ${tag.name}`);
+      }
+      names.add(tag.name);
+    }
+
     this.payload.tags = tags;
     return this;
   }
diff --git a/src/types.ts b/src/types.ts
index 4049259..41a7b0a 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -4,6 +4,12 @@
 
 export type MessageStatus = "scheduled" | "pending" | "queued" | "quarantined" | "suppressed" | "processed" | "delivered" | "opened" | "clicked" | "soft_bounced" | "hard_bounced" | "spam_complaint" | "failed" | "blocked" | "policy_rejected" | "unsubscribed" | "canceled";
 
+/** A reusable exact-match message tag. */
+export interface MessageTag {
+  "name": string;
+  "value": string;
+}
+
 export interface SendMailRequest {
   "route"?: string;
   "from": string;
@@ -16,10 +22,7 @@ export interface SendMailRequest {
   "headers"?: Record<string, string>;
   "metadata"?: Record<string, string>;
   "tag"?: string | null;
-  "tags"?: {
-  "name": string;
-  "value": string;
-}[];
+  "tags"?: MessageTag[];
   "settings"?: {
   "track_opens"?: boolean;
   "track_clicks"?: boolean;
@@ -47,10 +50,7 @@ export type SendBatchMailRequest = {
   "headers"?: Record<string, string>;
   "metadata"?: Record<string, string>;
   "tag"?: string | null;
-  "tags"?: {
-  "name": string;
-  "value": string;
-}[];
+  "tags"?: MessageTag[];
   "settings"?: {
   "track_opens"?: boolean;
   "track_clicks"?: boolean;
@@ -145,10 +145,7 @@ export interface MessageData {
   "status_changed_at": string | null;
   "scheduled_at": string | null;
   "tag": string | null;
-  "tags": {
-  "name": string;
-  "value": string;
-}[];
+  "tags": MessageTag[];
   "from_email": string;
   "from_name": string | null;
   "reply_to": string[] | null;
@@ -168,10 +165,7 @@ export interface MessageEventData {
   "message_id": string;
   "event": MessageEventType;
   "tag": string | null;
-  "tags": {
-  "name": string;
-  "value": string;
-}[];
+  "tags": MessageTag[];
   "metadata": Record<string, unknown> | null;
   "timestamp": string;
 }
@@ -192,10 +186,7 @@ export interface MessageListData {
   "bcc": MessageRecipientData[] | null;
   "reply_to": string[] | null;
   "tag": string | null;
-  "tags": {
-  "name": string;
-  "value": string;
-}[];
+  "tags": MessageTag[];
   "status_changed_at": string | null;
   "created_at": string;
 }