diff --git a/lib/resend/mailer.rb b/lib/resend/mailer.rb
index 5ec41ea..9bdc1f7 100644
--- a/lib/resend/mailer.rb
+++ b/lib/resend/mailer.rb
@@ -14,7 +14,7 @@ class Mailer
       cc bcc
       from reply-to to subject mime-version
       html text
-      content-type tags scheduled_at
+      content-type content-transfer-encoding tags scheduled_at
       headers options
     ].freeze
 
diff --git a/spec/railtie/mailer_spec.rb b/spec/railtie/mailer_spec.rb
index 9bfcfde..0e083a9 100644
--- a/spec/railtie/mailer_spec.rb
+++ b/spec/railtie/mailer_spec.rb
@@ -207,4 +207,13 @@ class TestMailerWithDisplayName < TestMailer
     body = @mailer.build_resend_params(message)
     expect(body[:headers].keys).to eq(["custom-header2"])
   end
+
+  it "filters content-transfer-encoding from custom headers" do
+    expect(Resend::Mailer::IGNORED_HEADERS).to include("content-transfer-encoding")
+
+    message = TestMailer.html_text_msg("test@example.org", "Test!")
+    body = @mailer.build_resend_params(message)
+    headers = body[:headers] || {}
+    expect(headers).not_to have_key("Content-Transfer-Encoding")
+  end
 end