diff --git a/web/src/store/__tests__/compose-email.test.ts b/web/src/store/__tests__/compose-email.test.ts
new file mode 100644
index 0000000..1424bcd
--- /dev/null
+++ b/web/src/store/__tests__/compose-email.test.ts
@@ -0,0 +1,76 @@
+import { beforeEach, describe, expect, it } from "vitest";
+import { buildEmailObject, type Draft } from "@/store/compose";
+import { useMail } from "@/store/mail";
+
+/**
+ * JMAP servers may reject `null` for a header property — Stalwart parses these
+ * as address lists and fails the whole create, which took down every send.
+ * Empty header fields must be omitted, not nulled.
+ */
+function draft(over: Partial<Draft> = {}): Draft {
+  return {
+    key: "k", draftId: null, identityId: "i1",
+    to: [{ name: null, email: "ann@example.com" }],
+    cc: [], bcc: [], replyTo: [],
+    subject: "Hello", html: "", text: "Hi there", format: "text",
+    attachments: [], inReplyTo: null, references: null,
+    relatedEmailId: null, relatedKeyword: null,
+    requestReceipt: false, priority: "normal",
+    showCc: false, showBcc: false, showReplyTo: false,
+    minimized: false, maximized: false, dirty: false, savedAt: null,
+    saving: false, sending: false, error: null, signatureHtml: "", replyMode: null,
+    ...over,
+  };
+}
+
+beforeEach(() => {
+  useMail.setState({
+    accountId: "a1",
+    identities: [{ id: "i1", name: "John", email: "john@example.org", replyTo: null }] as never,
+    mailboxes: { mb1: { id: "mb1", role: "sent", name: "Sent" }, mb2: { id: "mb2", role: "drafts", name: "Drafts" } } as never,
+  });
+});
+
+describe("buildEmailObject", () => {
+  it("omits empty header properties rather than sending null", async () => {
+    const obj = await buildEmailObject(draft(), { forSend: true });
+    expect(obj).not.toHaveProperty("cc");
+    expect(obj).not.toHaveProperty("bcc");
+    expect(obj).not.toHaveProperty("replyTo");
+    expect(obj).not.toHaveProperty("inReplyTo");
+    expect(obj).not.toHaveProperty("references");
+    expect(obj.to).toEqual([{ name: null, email: "ann@example.com" }]);
+  });
+
+  it("includes header properties that have a value", async () => {
+    const obj = await buildEmailObject(
+      draft({
+        cc: [{ name: null, email: "c@x.io" }],
+        bcc: [{ name: null, email: "d@x.io" }],
+        replyTo: [{ name: null, email: "r@x.io" }],
+        inReplyTo: ["<a@b>"],
+        references: ["<a@b>"],
+      }),
+      { forSend: true },
+    );
+    expect(obj.cc).toHaveLength(1);
+    expect(obj.bcc).toHaveLength(1);
+    expect(obj.replyTo).toHaveLength(1);
+    expect(obj.inReplyTo).toEqual(["<a@b>"]);
+    expect(obj.references).toEqual(["<a@b>"]);
+  });
+
+  it("never emits a null value for any property", async () => {
+    for (const forSend of [true, false]) {
+      const obj = await buildEmailObject(draft({ subject: "" }), { forSend });
+      for (const [k, v] of Object.entries(obj)) {
+        expect(v, `${k} is null`).not.toBeNull();
+      }
+    }
+  });
+
+  it("files a sent message in Sent and a draft in Drafts", async () => {
+    expect((await buildEmailObject(draft(), { forSend: true })).mailboxIds).toEqual({ mb1: true });
+    expect((await buildEmailObject(draft(), { forSend: false })).mailboxIds).toEqual({ mb2: true });
+  });
+});
diff --git a/web/src/store/compose.ts b/web/src/store/compose.ts
index 4e18300..1702573 100644
--- a/web/src/store/compose.ts
+++ b/web/src/store/compose.ts
@@ -481,7 +481,7 @@ function scheduleAutosave(key: string, get: () => ComposeState) {
 }
 
 /** Build the JMAP Email creation object from a draft. */
-async function buildEmailObject(d: Draft, opts: { forSend: boolean }): Promise<Record<string, unknown>> {
+export async function buildEmailObject(d: Draft, opts: { forSend: boolean }): Promise<Record<string, unknown>> {
   const mail = useMail.getState();
   const accountId = mail.accountId!;
   const ident = mail.identities.find((i) => i.id === d.identityId) ?? mail.identities[0];
@@ -561,18 +561,23 @@ async function buildEmailObject(d: Draft, opts: { forSend: boolean }): Promise<R
 
   const obj: Record<string, unknown> = {
     from: [from],
-    to: d.to.length ? d.to : null,
-    cc: d.cc.length ? d.cc : null,
-    bcc: d.bcc.length ? d.bcc : null,
-    replyTo: d.replyTo.length ? d.replyTo : ident.replyTo?.length ? ident.replyTo : null,
     subject: d.subject,
     sentAt: new Date().toISOString().replace(/\.\d{3}Z$/, "Z"),
-    inReplyTo: d.inReplyTo,
-    references: d.references,
     bodyStructure,
     bodyValues,
     "header:User-Agent:asText": "ihasmail/2.0",
   };
+  // Header properties are omitted when empty, never sent as null: JMAP servers
+  // are entitled to reject null for a header field (Stalwart parses these as
+  // address lists and fails the whole create), and on a create there is no
+  // previous value that would need clearing.
+  const replyTo = d.replyTo.length ? d.replyTo : (ident.replyTo ?? []);
+  if (d.to.length) obj.to = d.to;
+  if (d.cc.length) obj.cc = d.cc;
+  if (d.bcc.length) obj.bcc = d.bcc;
+  if (replyTo.length) obj.replyTo = replyTo;
+  if (d.inReplyTo?.length) obj.inReplyTo = d.inReplyTo;
+  if (d.references?.length) obj.references = d.references;
   if (d.priority === "high") {
     obj["header:X-Priority:asText"] = "1 (Highest)";
     obj["header:Importance:asText"] = "High";