diff --git a/lib/courrier/email/providers/mailgun.rb b/lib/courrier/email/providers/mailgun.rb
index 7afb3ca..a2f74f3 100644
--- a/lib/courrier/email/providers/mailgun.rb
+++ b/lib/courrier/email/providers/mailgun.rb
@@ -33,7 +33,7 @@ def content_type = "multipart/form-data"
 
         def default_headers
           {
-            "Authorization" => "Basic #{Base64.strict_encode64("api:#{@api_key}")}"
+            "Authorization" => "Basic #{["api:#{@api_key}"].pack("m0")}"
           }
         end
       end
diff --git a/lib/courrier/email/providers/mailjet.rb b/lib/courrier/email/providers/mailjet.rb
index 2ffeb44..fe43574 100644
--- a/lib/courrier/email/providers/mailjet.rb
+++ b/lib/courrier/email/providers/mailjet.rb
@@ -35,7 +35,7 @@ def body
 
         def default_headers
           {
-            "Authorization" => "Basic " + Base64.strict_encode64("#{@api_key}:#{@provider_options.api_secret}")
+            "Authorization" => "Basic #{["#{@api_key}:#{@provider_options.api_secret}"].pack("m0")}"
           }
         end
 
diff --git a/test/courrier/email/providers/mailgun_test.rb b/test/courrier/email/providers/mailgun_test.rb
new file mode 100644
index 0000000..7374313
--- /dev/null
+++ b/test/courrier/email/providers/mailgun_test.rb
@@ -0,0 +1,86 @@
+require "test_helper"
+
+module Courrier::Email::Providers
+  class MailgunTest < Minitest::Test
+    def setup
+      email = TestEmail.new(
+        from: "devs@railsdesigner.com",
+        to: "first@example.com, second@example.com",
+        reply_to: "support@railsdesigner.com"
+      )
+
+      @provider = provider_for(email)
+    end
+
+    def test_formats_transactional_email
+      assert_equal(
+        {
+          "from" => "devs@railsdesigner.com",
+          "to" => "first@example.com, second@example.com",
+          "h:Reply-To" => "support@railsdesigner.com",
+          "subject" => "Test Subject",
+          "text" => "Test Body",
+          "html" => "<p>Test HTML Body</p>"
+        },
+        @provider.body
+      )
+    end
+
+    def test_omits_reply_to_when_not_set
+      body = provider_for(TestEmail.new(from: "devs@railsdesigner.com", to: "first@example.com")).body
+
+      refute_includes body.keys, "h:Reply-To"
+    end
+
+    def test_builds_endpoint_url_from_domain
+      assert_equal "https://api.mailgun.net/v3/railsdesigner.com/messages", @provider.send(:endpoint_url)
+    end
+
+    def test_requires_a_domain
+      provider = Mailgun.new(
+        api_key: "test_key",
+        options: TestEmail.new(from: "devs@railsdesigner.com", to: "first@example.com").options,
+        provider_options: Courrier::Configuration::ProviderConfig.new
+      )
+
+      error = assert_raises(Courrier::ArgumentError) { provider.send(:endpoint_url) }
+
+      assert_match(/domain/, error.message)
+    end
+
+    def test_authenticates_with_api_key
+      assert_equal(
+        {"Authorization" => "Basic YXBpOnRlc3Rfa2V5"},
+        @provider.send(:default_headers)
+      )
+    end
+
+    def test_submits_as_multipart_form_data
+      assert_equal "multipart/form-data", @provider.send(:content_type)
+    end
+
+    def test_is_available_through_provider_registry
+      mock_provider = Minitest::Mock.new
+      mock_provider.expect(:deliver, nil)
+
+      Mailgun.stub :new, mock_provider do
+        Courrier::Email::Provider.new(
+          provider: "mailgun",
+          api_key: "test_key",
+          options: @provider.instance_variable_get(:@options)
+        ).deliver
+      end
+
+      mock_provider.verify
+    end
+
+    private
+
+    def provider_for(email)
+      provider_options = Courrier::Configuration::ProviderConfig.new
+      provider_options.domain = "railsdesigner.com"
+
+      Mailgun.new(api_key: "test_key", options: email.options, provider_options: provider_options)
+    end
+  end
+end
diff --git a/test/courrier/email/providers/mailjet_test.rb b/test/courrier/email/providers/mailjet_test.rb
new file mode 100644
index 0000000..d02f052
--- /dev/null
+++ b/test/courrier/email/providers/mailjet_test.rb
@@ -0,0 +1,70 @@
+require "test_helper"
+
+module Courrier::Email::Providers
+  class MailjetTest < Minitest::Test
+    def setup
+      email = TestEmail.new(
+        from: "devs@railsdesigner.com",
+        to: "first@example.com",
+        reply_to: "support@railsdesigner.com"
+      )
+
+      @provider = provider_for(email)
+    end
+
+    def test_formats_transactional_email
+      assert_equal(
+        {
+          "Messages" => [
+            {
+              "From" => {"Email" => "devs@railsdesigner.com"},
+              "To" => [{"Email" => "first@example.com"}],
+              "ReplyTo" => {"Email" => "support@railsdesigner.com"},
+              "Subject" => "Test Subject",
+              "TextPart" => "Test Body",
+              "HTMLPart" => "<p>Test HTML Body</p>"
+            }
+          ]
+        },
+        @provider.body
+      )
+    end
+
+    def test_omits_reply_to_when_not_set
+      body = provider_for(TestEmail.new(from: "devs@railsdesigner.com", to: "first@example.com")).body
+
+      refute_includes body["Messages"].first.keys, "ReplyTo"
+    end
+
+    def test_authenticates_with_api_key_and_secret
+      assert_equal(
+        {"Authorization" => "Basic dGVzdF9rZXk6dGVzdF9zZWNyZXQ="},
+        @provider.send(:default_headers)
+      )
+    end
+
+    def test_is_available_through_provider_registry
+      mock_provider = Minitest::Mock.new
+      mock_provider.expect(:deliver, nil)
+
+      Mailjet.stub :new, mock_provider do
+        Courrier::Email::Provider.new(
+          provider: "mailjet",
+          api_key: "test_key",
+          options: @provider.instance_variable_get(:@options)
+        ).deliver
+      end
+
+      mock_provider.verify
+    end
+
+    private
+
+    def provider_for(email)
+      provider_options = Courrier::Configuration::ProviderConfig.new
+      provider_options.api_secret = "test_secret"
+
+      Mailjet.new(api_key: "test_key", options: email.options, provider_options: provider_options)
+    end
+  end
+end