test: add smtp sender test for MX lookup timeouts

spec/senders/smtp_sender_spec.rb

@@ -46,6 +46,7 @@ RSpec.describe SMTPSender do
 
   describe "#start" do
     context "when no servers are provided to the class and there are no SMTP relays" do
+      context "when there are MX records" do
         before do
           allow(DNSResolver.local).to receive(:mx).and_return([[5, "mx1.example.com"], [10, "mx2.example.com"]])
           allow(DNSResolver.local).to receive(:a).with("mx1.example.com").and_return(["1.2.3.4"])
@@ -62,6 +63,34 @@ RSpec.describe SMTPSender do
         end
       end
 
+      context "when there are no MX records" do
+        before do
+          allow(DNSResolver.local).to receive(:mx).and_return([])
+          allow(DNSResolver.local).to receive(:a).with("example.com").and_return(["1.2.3.4"])
+        end
+
+        it "attempts to create an SMTP connection for the domain itself" do
+          endpoint = sender.start
+          expect(endpoint).to be_a SMTPClient::Endpoint
+          expect(endpoint).to have_attributes(
+            ip_address: "1.2.3.4",
+            server: have_attributes(hostname: "example.com", port: 25, ssl_mode: SMTPClient::SSLModes::AUTO)
+          )
+        end
+      end
+
+      context "when the MX lookup times out" do
+        before do
+          allow(DNSResolver.local).to receive(:mx).and_raise(Resolv::ResolvError.new("DNS resolv timeout: example.com"))
+          allow(DNSResolver.local).to receive(:a).with("example.com").and_return(["1.2.3.4"])
+        end
+
+        it "raises an error" do
+          expect { sender.start }.to raise_error Resolv::ResolvError
+        end
+      end
+    end
+
     context "when there are no servers provided to the class but there are SMTP relays" do
       before do
         allow(SMTPSender).to receive(:smtp_relays).and_return([SMTPClient::Server.new("relay.example.com", port: 2525, ssl_mode: SMTPClient::SSLModes::TLS)])