Add temporary failure condition in client test

Introduce a new `FailTemp` property to simulate a temporary failure during the DATA transmission phase in tests. This helps ensure better coverage and handling of specific error conditions.

client_test.go

@@ -3559,6 +3559,7 @@ type serverProps struct {
 	FailOnQuit      bool
 	FailOnReset     bool
 	FailOnSTARTTLS  bool
+	FailTemp        bool
 	FeatureSet      string
 	ListenPort      int
 	SSLListener     bool
@@ -3720,6 +3721,10 @@ func handleTestServerConnection(connection net.Conn, t *testing.T, props *server
 						writeLine("500 5.0.0 Error during DATA transmission")
 						break
 					}
+					if props.FailTemp {
+						writeLine("451 4.3.0 Error: fail on DATA close")
+						break
+					}
 					writeLine("250 2.0.0 Ok: queued as 1234567890")
 					break
 				}