diff --git a/lib/net/imap/command_data.rb b/lib/net/imap/command_data.rb
index 9a5749b5..d0a5fc51 100644
--- a/lib/net/imap/command_data.rb
+++ b/lib/net/imap/command_data.rb
@@ -85,15 +85,23 @@ def send_binary_literal(*, **) = send_literal(*, **, binary: true)
 
     # `non_sync` is an optional tri-state flag:
     # * `true`  -> Force non-synchronizing `LITERAL+`/`LITERAL-` behavior.
-    #   TODO: raise or warn when capabilities don't allow non_sync.
+    #   NOTE: raises DataFormatError when server doesn't support
+    #   non-synchronizing literal, or literal is too large for LITERAL-.
     # * `false` -> Force normal synchronizing literal behavior.
     # * `nil`   -> (default) Currently behaves like `false` (will be dynamic).
     def send_literal(str, tag = nil, binary: false, non_sync: nil)
+      bytesize = str.bytesize
       synchronize do
-        non_sync = non_sync_literal?(str.bytesize) if non_sync.nil?
+        if non_sync && !non_sync_literal_allowed?(bytesize)
+          # TODO: check in Printer, so we don't need to close the connection.
+          @sock.close
+          raise DataFormatError, "Connection closed: " \
+            "Cannot send non-synchronizing literal without known server support"
+        end
+        non_sync = non_sync_literal?(bytesize) if non_sync.nil?
         prefix = "~" if binary
         plus = "+" if non_sync
-        put_string("#{prefix}{#{str.bytesize}#{plus}}\r\n")
+        put_string("#{prefix}{#{bytesize}#{plus}}\r\n")
         if non_sync
           put_string(str)
           return
@@ -113,12 +121,19 @@ def send_literal(str, tag = nil, binary: false, non_sync: nil)
     end
 
     def non_sync_literal?(bytesize)
-      capabilities_cached? &&
-        bytesize <= config.max_non_synchronizing_literal &&
-        (capable?("LITERAL+") ||
-         bytesize <= 4096 && (capable?("IMAP4rev2") || capable?("LITERAL-")))
+      bytesize <= config.max_non_synchronizing_literal \
+        && non_sync_literal_allowed?(bytesize)
+    end
+
+    def non_sync_literal_allowed?(bytesize)
+      return unless capabilities_cached?
+      return "+" if capable?("LITERAL+")
+      return "-" if capable_literal_minus? && bytesize <= 4096
+      false
     end
 
+    def capable_literal_minus? = capable?("LITERAL-") || capable?("IMAP4rev2")
+
     # NOTE: +num+ should already be an Integer
     def send_number_data(num)
       put_string(Integer(num).to_s)
diff --git a/test/net/imap/test_imap.rb b/test/net/imap/test_imap.rb
index f43bdcc7..321cc258 100644
--- a/test/net/imap/test_imap.rb
+++ b/test/net/imap/test_imap.rb
@@ -869,6 +869,55 @@ def test_raw_data
     end
   end
 
+  test("send non-synchronizing literals with LITERAL+") do
+    with_fake_server(
+      with_extensions: %w[LITERAL+], greeting_capabilities: true,
+    ) do |server, imap|
+      def imap.send_test_args(*args) = send_command("TEST", *args)
+      server.on "TEST", &:done_ok
+
+      imap.config.max_non_synchronizing_literal = 5_000
+      large = "\xff".b * 5_000
+      imap.send_test_args Net::IMAP::Literal[large, nil]
+      assert_equal("{5000+}\r\n#{large}".b, server.commands.pop.args)
+
+      large = "\xff".b * 10_000
+      imap.send_test_args Net::IMAP::Literal[large, nil]
+      assert_equal("{10000}\r\n#{large}".b, server.commands.pop.args)
+
+      imap.send_test_args Net::IMAP::Literal[large, true]
+      assert_equal("{10000+}\r\n#{large}".b, server.commands.pop.args)
+    end
+  end
+
+  test("send non-synchronizing literal that's too large for LITERAL-") do
+    with_fake_server(
+      with_extensions: %w[LITERAL-], greeting_capabilities: true,
+      ignore_abrupt_eof: true, ignore_io_error: true
+    ) do |server, imap|
+      def imap.send_test_args(*args) = send_command("TEST", *args)
+      server.on "TEST", &:done_ok
+      assert_raise(Net::IMAP::DataFormatError) do
+        imap.send_test_args Net::IMAP::Literal["\xff".b * 5000, true]
+      end
+      assert imap.disconnected?
+    end
+  end
+
+  test("send non-synchronizing literal without known server support") do
+    with_fake_server(
+      with_extensions: %w[LITERAL+], greeting_capabilities: false,
+      ignore_abrupt_eof: true, ignore_io_error: true
+    ) do |server, imap|
+      def imap.send_test_args(*args) = send_command("TEST", *args)
+      server.on "TEST", &:done_ok
+      assert_raise(Net::IMAP::DataFormatError) do
+        imap.send_test_args Net::IMAP::Literal["\xff".b * 100, true]
+      end
+      assert imap.disconnected?
+    end
+  end
+
   def test_disconnect
     server = create_tcp_server
     port = server.addr[1]