diff --git a/lib/resend/swoosh/adapter.ex b/lib/resend/swoosh/adapter.ex
index 4cca05f..7288edd 100644
--- a/lib/resend/swoosh/adapter.ex
+++ b/lib/resend/swoosh/adapter.ex
@@ -68,11 +68,13 @@ defmodule Resend.Swoosh.Adapter do
   end
 
   defp format_attachment(%Swoosh.Attachment{} = attachment) do
+    cid = if is_nil(attachment.cid), do: attachment.filename, else: attachment.cid
+
     %Resend.Emails.Attachment{
       content: Swoosh.Attachment.get_content(attachment, :base64),
       content_type: attachment.content_type,
       filename: attachment.filename,
-      content_id: attachment.cid
+      content_id: cid
     }
   end
 
diff --git a/test/resend/emails_test.exs b/test/resend/emails_test.exs
index 14c2e63..32e1678 100644
--- a/test/resend/emails_test.exs
+++ b/test/resend/emails_test.exs
@@ -179,6 +179,41 @@ defmodule Resend.EmailsTest do
       assert {:ok, _} = Resend.Swoosh.Adapter.deliver(email, config)
     end
 
+    test "Swoosh adapter uses filename as content_id when cid is not set", context do
+      email =
+        Swoosh.Email.new()
+        |> Swoosh.Email.to(context.to)
+        |> Swoosh.Email.from(context.from)
+        |> Swoosh.Email.subject("Test Inline Image")
+        |> Swoosh.Email.html_body(~s|<img src="cid:logo.jpg">|)
+        |> Swoosh.Email.attachment(
+          Swoosh.Attachment.new(
+            {:data, "fake_binary_data"},
+            filename: "logo.jpg",
+            content_type: "image/jpeg",
+            type: :inline
+          )
+        )
+
+      Req.Test.stub(Resend.ReqStub, fn conn ->
+        assert conn.method == "POST"
+        assert conn.request_path == "/emails"
+
+        {:ok, body, conn} = Plug.Conn.read_body(conn)
+        body = Jason.decode!(body)
+
+        assert [attachment] = body["attachments"]
+        assert attachment["content_id"] == "logo.jpg"
+
+        conn
+        |> Plug.Conn.put_resp_content_type("application/json")
+        |> Plug.Conn.send_resp(200, Jason.encode!(%{"id" => context.sent_email_id}))
+      end)
+
+      config = [api_key: context.api_key]
+      assert {:ok, _} = Resend.Swoosh.Adapter.deliver(email, config)
+    end
+
     test "list/1 lists all sent emails", context do
       ClientMock.mock_request(context,
         method: :get,