diff --git a/lib/net/smtp.rb b/lib/net/smtp.rb
index cca06e6..8567da5 100644
--- a/lib/net/smtp.rb
+++ b/lib/net/smtp.rb
@@ -300,7 +300,9 @@ def capable_starttls?
     # true if the EHLO response contains +key+.
     def capable?(key)
       return nil unless @capabilities
-      @capabilities[key] ? true : false
+      return true if @capabilities[key]
+      key = key.to_s
+      @capabilities.each_key.any? { |capability| capability&.casecmp?(key) }
     end
 
     # The server capabilities by EHLO response
diff --git a/test/net/smtp/test_smtp.rb b/test/net/smtp/test_smtp.rb
index 3b9e245..80d339d 100644
--- a/test/net/smtp/test_smtp.rb
+++ b/test/net/smtp/test_smtp.rb
@@ -51,6 +51,7 @@ def test_server_capabilities
         smtp = Net::SMTP.start('localhost', port, starttls: false)
         assert_equal({"STARTTLS"=>[], "AUTH"=>["PLAIN"]}, smtp.capabilities)
         assert_equal(true, smtp.capable?('STARTTLS'))
+        assert_equal(true, smtp.capable?('starttls'))
         assert_equal(false, smtp.capable?('DOES-NOT-EXIST'))
       else
         port = fake_server_start