diff --git a/conn.go b/conn.go
index ff43e0ed..df3870c2 100644
--- a/conn.go
+++ b/conn.go
@@ -23,10 +23,11 @@ import (
 const errThreshold = 3
 
 type Conn struct {
-	conn   net.Conn
-	text   *textproto.Conn
-	server *Server
-	helo   string
+	conn     net.Conn
+	text     *textproto.Conn
+	server   *Server
+	helo     string
+	rejected bool
 
 	// Number of errors witnessed on this connection
 	errCount int
@@ -105,6 +106,17 @@ func (c *Conn) handle(cmd string, arg string) {
 		return
 	}
 
+	//as per RFC5321 3.1
+	if c.rejected {
+		if cmd == "QUIT" {
+			c.writeResponse(221, NoEnhancedCode, "OK")
+			c.Close()
+		} else {
+			c.protocolError(503, NoEnhancedCode, "bad sequence of commands")
+		}
+		return
+	}
+
 	cmd = strings.ToUpper(cmd)
 	switch cmd {
 	case "SEND", "SOML", "SAML", "EXPN", "HELP", "TURN":
@@ -1276,7 +1288,13 @@ func (c *Conn) greet() {
 	if c.server.LMTP {
 		protocol = "LMTP"
 	}
-	c.writeResponse(220, NoEnhancedCode, fmt.Sprintf("%v %s Service Ready", c.server.Domain, protocol))
+	domain, err := c.server.GetDomain(c)
+	if err != nil {
+		c.writeResponse(554, NoEnhancedCode, "Error: "+err.Error())
+		c.rejected = true
+		return
+	}
+	c.writeResponse(220, NoEnhancedCode, fmt.Sprintf("%v %s Service Ready", domain, protocol))
 }
 
 func (c *Conn) writeResponse(code int, enhCode EnhancedCode, text ...string) {
diff --git a/server.go b/server.go
index e0e0acd0..8a32754c 100644
--- a/server.go
+++ b/server.go
@@ -40,6 +40,7 @@ type Server struct {
 	ErrorLog          Logger
 	ReadTimeout       time.Duration
 	WriteTimeout      time.Duration
+	domainFn          func(*Conn) (string, error)
 
 	// Advertise SMTPUTF8 (RFC 6531) capability.
 	// Should be used only if backend supports it.
@@ -148,6 +149,19 @@ func (s *Server) Serve(l net.Listener) error {
 	}
 }
 
+// Getter for the optional dynamic domain string generator function
+func (s *Server) GetDomain(c *Conn) (string, error) {
+	if s.domainFn != nil {
+		return s.domainFn(c)
+	}
+	return s.Domain, nil
+}
+
+// Setter for the dynamic domain string generator function
+func (s *Server) SetDomainFunc(fn func(*Conn) (string, error)) {
+	s.domainFn = fn
+}
+
 func (s *Server) handleConn(c *Conn) error {
 	s.locker.Lock()
 	s.conns[c] = struct{}{}
diff --git a/server_test.go b/server_test.go
index 06db7468..1febebfe 100644
--- a/server_test.go
+++ b/server_test.go
@@ -1712,3 +1712,95 @@ func TestServerMTPRIORITY(t *testing.T) {
 		t.Fatal("Incorrect MtPriority parameter value:", fmt.Sprintf("expected %d, got %d", expectedPriority, *priority))
 	}
 }
+
+func getDynamicDomainResponse(c *smtp.Conn) (string, error) {
+	return "dynamichost.local", nil
+}
+
+func getNegativeDynamicDomainResponse(c *smtp.Conn) (string, error) {
+	return "", fmt.Errorf("no service")
+}
+
+func testServerDynamicDomain(t *testing.T, fn ...serverConfigureFunc) (be *backend, s *smtp.Server, c net.Conn, scanner *bufio.Scanner) {
+	l, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	be = new(backend)
+	s = smtp.NewServer(be)
+	s.Domain = "localhost"
+	s.SetDomainFunc(getDynamicDomainResponse)
+	s.AllowInsecureAuth = true
+	for _, f := range fn {
+		f(s)
+	}
+
+	go s.Serve(l)
+
+	c, err = net.Dial("tcp", l.Addr().String())
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	scanner = bufio.NewScanner(c)
+	return
+}
+
+func testServerNegativeDynamicDomain(t *testing.T, fn ...serverConfigureFunc) (be *backend, s *smtp.Server, c net.Conn, scanner *bufio.Scanner) {
+	l, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	be = new(backend)
+	s = smtp.NewServer(be)
+	s.Domain = "localhost"
+	s.SetDomainFunc(getNegativeDynamicDomainResponse)
+	s.AllowInsecureAuth = true
+	for _, f := range fn {
+		f(s)
+	}
+
+	go s.Serve(l)
+
+	c, err = net.Dial("tcp", l.Addr().String())
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	scanner = bufio.NewScanner(c)
+	return
+}
+
+func TestServerDynamicDomainGreeted(t *testing.T) {
+	_, _, _, scanner := testServerDynamicDomain(t)
+
+	scanner.Scan()
+	if scanner.Text() != "220 dynamichost.local ESMTP Service Ready" {
+		t.Fatal("Invalid greeting:", scanner.Text())
+	}
+}
+
+func TestServerNegativeDynamicDomainGreeted(t *testing.T) {
+	_, _, c, scanner := testServerNegativeDynamicDomain(t)
+
+	scanner.Scan()
+	if scanner.Text() != "554 Error: no service" {
+		t.Fatal("Invalid greeting:", scanner.Text())
+	}
+
+	//Now test for 503 error as per RFC521 3.1
+	io.WriteString(c, "HELO localhost\r\n")
+
+	scanner.Scan()
+	if !strings.HasPrefix(scanner.Text(), "503 bad sequence of commands") {
+		t.Fatal("Invalid HELO response:", scanner.Text())
+	}
+	io.WriteString(c, "QUIT\r\n")
+
+	scanner.Scan()
+	if !strings.HasPrefix(scanner.Text(), "221 OK") {
+		t.Fatal("Invalid HELO response:", scanner.Text())
+	}
+}