diff --git a/lib/net/smtp.rb b/lib/net/smtp.rb
index cca06e6..d77f037 100644
--- a/lib/net/smtp.rb
+++ b/lib/net/smtp.rb
@@ -1104,7 +1104,7 @@ def capabilities
         h = {}
         @string.lines.drop(1).each do |line|
           k, *v = line[4..-1].split(' ')
-          h[k] = v
+          h[k&.upcase] = v
         end
         h
       end
diff --git a/test/net/smtp/test_response.rb b/test/net/smtp/test_response.rb
index 3cf909a..84da1c1 100644
--- a/test/net/smtp/test_response.rb
+++ b/test/net/smtp/test_response.rb
@@ -24,6 +24,15 @@ def test_capabilities_value
         assert_equal ['1234'], res.capabilities['SIZE']
       end
 
+      def test_capabilities_normalize_keyword_case_only
+        res = Response.parse("250-fixture\n250-smtputf8\n250 size MiXeD\n")
+        assert_equal({
+                       "SMTPUTF8" => [],
+                       "SIZE" => ["MiXeD"],
+                     },
+                     res.capabilities)
+      end
+
       def test_capabilities_multi
         res = Response.parse("250-ubuntu-desktop\n250-SIZE 1 2 3\n250 DSN\n")
         assert_equal %w{1 2 3}, res.capabilities['SIZE']