Implement batch processing for sendMany()

Instead of sending emails sequentially, sendMany() now batches multiple
messages into a single JMAP request with multiple Email/set creates and
EmailSubmission/set creates. This reduces HTTP round-trips and improves
throughput when sending multiple emails.

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

Author: dahlia

docs/transports/jmap.md

@@ -187,7 +187,7 @@ Bulk email sending
 ------------------
 
 For sending multiple emails, the JMAP transport provides efficient
-sequential processing with comprehensive error handling:
+batch processing that combines all messages into a single HTTP request:
 
 ~~~~ typescript twoslash
 import { JmapTransport } from "@upyo/jmap";
@@ -224,10 +224,10 @@ for await (const receipt of transport.sendMany(messages)) {
 }
 ~~~~
 
-The `~JmapTransport.sendMany()` method processes emails sequentially,
-yielding a receipt for each message. Failed emails don't prevent
-subsequent emails from being sent, and the session is efficiently
-reused across all messages.
+The `~JmapTransport.sendMany()` method batches all emails into a single
+JMAP request, significantly reducing HTTP round-trips. Each message
+gets its own receipt, and partial failures are handled gracefully—if
+some emails fail, others in the batch can still succeed.
 
 
 Error handling

packages/jmap/src/jmap-transport.e2e.test.ts

@@ -230,7 +230,7 @@ describe(
       assert.ok(receipt.successful);
     });
 
-    it("should send multiple emails with sendMany", async () => {
+    it("should send multiple emails with sendMany in a single batch", async () => {
       const config = getTestConfig();
       const transport = new JmapTransport(config.jmap);
 
@@ -243,6 +243,10 @@ describe(
           subject: "E2E Test - Batch 2",
           content: { text: "Second email in batch" },
         }),
+        createTestMessage({
+          subject: "E2E Test - Batch 3",
+          content: { text: "Third email in batch" },
+        }),
       ];
 
       const receipts = [];
@@ -253,6 +257,69 @@ describe(
         receipts.push(receipt);
       }
 
+      assert.equal(receipts.length, 3);
+      assert.ok(receipts.every((r) => r.successful));
+
+      // Verify each message has a unique submission ID
+      if (receipts.every((r) => r.successful)) {
+        const messageIds = receipts.map((r) =>
+          r.successful ? r.messageId : null
+        );
+        const uniqueIds = new Set(messageIds);
+        assert.equal(
+          uniqueIds.size,
+          3,
+          "Each message should have a unique submission ID",
+        );
+      }
+    });
+
+    it("should batch emails with attachments via sendMany", async () => {
+      const config = getTestConfig();
+      const transport = new JmapTransport(config.jmap);
+
+      const textBytes = new TextEncoder().encode("Attachment content");
+
+      const messages = [
+        createTestMessage({
+          subject: "E2E Test - Batch with Attachment 1",
+          content: { text: "First email with attachment" },
+          attachments: [
+            {
+              filename: "file1.txt",
+              contentType: "text/plain" as const,
+              content: textBytes,
+              contentId: "attachment-1",
+              inline: false,
+            },
+          ],
+        }),
+        createTestMessage({
+          subject: "E2E Test - Batch with Attachment 2",
+          content: { text: "Second email with attachment" },
+          attachments: [
+            {
+              filename: "file2.txt",
+              contentType: "text/plain" as const,
+              content: textBytes,
+              contentId: "attachment-2",
+              inline: false,
+            },
+          ],
+        }),
+      ];
+
+      const receipts = [];
+      for await (const receipt of transport.sendMany(messages)) {
+        if (!receipt.successful) {
+          console.error(
+            "sendMany with attachments failed:",
+            receipt.errorMessages,
+          );
+        }
+        receipts.push(receipt);
+      }
+
       assert.equal(receipts.length, 2);
       assert.ok(receipts.every((r) => r.successful));
     });