endpoint/smtp: Allow to run filters before session init

Mostly used for checks that indicate 'obviously malicious' clients so it
is preferable to immediately reject all commands without session
initialization and running other checks.

msgpipeline/msgpipeline.go

@@ -50,6 +50,22 @@ func New(globals map[string]interface{}, cfg []config.Node) (*MsgPipeline, error
 	}, err
 }
 
+func (d *MsgPipeline) RunEarlyChecks(state *smtp.ConnectionState) error {
+	// TODO: See if there is some point in parallelization of this
+	// function.
+	for _, check := range d.globalChecks {
+		earlyCheck, ok := check.(module.EarlyCheck)
+		if !ok {
+			continue
+		}
+
+		if err := earlyCheck.CheckConnection(state); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
 // Start starts new message delivery, runs connection and sender checks, sender modifiers
 // and selects source block from config to use for handling.
 //