diff --git a/packages/lib/getReplyToHeader.test.ts b/packages/lib/getReplyToHeader.test.ts
new file mode 100644
index 0000000000..0b558d948a
--- /dev/null
+++ b/packages/lib/getReplyToHeader.test.ts
@@ -0,0 +1,92 @@
+import { describe, expect, it, vi } from "vitest";
+
+import { getReplyToHeader } from "./getReplyToHeader";
+
+/**
+ * RFC 5322 (Internet Message Format) specifies that the Reply-To header must be
+ * a comma-separated list of addresses, not an array. Many SMTP servers reject arrays.
+ * Spec: https://datatracker.ietf.org/doc/html/rfc5322#section-3.6.2
+ */
+
+vi.mock("./getReplyToEmail", () => ({
+  getReplyToEmail: vi.fn((calEvent, excludeOrganizerEmail) => {
+    if (excludeOrganizerEmail) return null;
+    return calEvent.organizer?.email || null;
+  }),
+}));
+
+const createMockCalEvent = (organizerEmail: string) => ({
+  organizer: { email: organizerEmail },
+  hideOrganizerEmail: false,
+});
+
+describe("getReplyToHeader", () => {
+  describe("return type", () => {
+    it("always returns replyTo as a string, never an array", () => {
+      const calEvent = createMockCalEvent("organizer@test.com");
+      const result = getReplyToHeader(calEvent as any, ["attendee1@test.com", "attendee2@test.com"]);
+
+      expect(result).toHaveProperty("replyTo");
+      expect(typeof result.replyTo).toBe("string");
+      // Should NOT be an array
+      expect(Array.isArray(result.replyTo)).toBe(false);
+    });
+  });
+
+  describe("with single email", () => {
+    it("returns single email as string", () => {
+      const calEvent = createMockCalEvent("organizer@test.com");
+      const result = getReplyToHeader(calEvent as any);
+
+      expect(result).toEqual({ replyTo: "organizer@test.com" });
+    });
+
+    it("returns additionalEmail as string when provided alone", () => {
+      const calEvent = createMockCalEvent("organizer@test.com");
+      const result = getReplyToHeader(calEvent as any, "additional@test.com", true);
+
+      expect(result).toEqual({ replyTo: "additional@test.com" });
+    });
+  });
+
+  describe("with multiple emails", () => {
+    it("returns comma-separated string for multiple emails", () => {
+      const calEvent = createMockCalEvent("organizer@test.com");
+      const result = getReplyToHeader(calEvent as any, ["attendee1@test.com", "attendee2@test.com"]);
+
+      expect(result).toEqual({
+        replyTo: "attendee1@test.com, attendee2@test.com, organizer@test.com",
+      });
+    });
+
+    it("returns comma-separated string when additionalEmails is array", () => {
+      const calEvent = createMockCalEvent("organizer@test.com");
+      const result = getReplyToHeader(calEvent as any, ["a@test.com", "b@test.com", "c@test.com"], true);
+
+      expect(result).toEqual({
+        replyTo: "a@test.com, b@test.com, c@test.com",
+      });
+    });
+  });
+
+  describe("with no emails", () => {
+    it("returns empty object when no emails available", () => {
+      const calEvent = { organizer: { email: "" }, hideOrganizerEmail: true };
+      const result = getReplyToHeader(calEvent as any, undefined, true);
+
+      expect(result).toEqual({});
+    });
+  });
+
+  describe("SMTP compatibility", () => {
+    it("produces RFC 5322 compliant Reply-To header format", () => {
+      const calEvent = createMockCalEvent("organizer@test.com");
+      const result = getReplyToHeader(calEvent as any, ["a@test.com", "b@test.com"]);
+
+      // RFC 5322 specifies comma-separated list for multiple addresses
+      expect(result.replyTo).toMatch(/^[^,]+, [^,]+, [^,]+$/);
+      expect(result.replyTo).not.toContain("[");
+      expect(result.replyTo).not.toContain("]");
+    });
+  });
+});
diff --git a/packages/lib/getReplyToHeader.ts b/packages/lib/getReplyToHeader.ts
index 36254dfe54..cff6e458e1 100644
--- a/packages/lib/getReplyToHeader.ts
+++ b/packages/lib/getReplyToHeader.ts
@@ -26,6 +26,6 @@ export function getReplyToHeader(
     return {};
   }
 
-  const replyTo = emailArray.length === 1 ? emailArray[0] : emailArray;
+  const replyTo = emailArray.join(", ");
   return { replyTo };
 }