diff --git a/server.go b/server.go
index 98702bf5..4c8739de 100644
--- a/server.go
+++ b/server.go
@@ -377,6 +377,7 @@ func (server *server) handleClient(client *client) {
 		advertiseTLS = ""
 	}
 
+	firstCmd := true
 	for client.isAlive() {
 		switch client.state {
 		case ClientGreeting:
@@ -413,6 +414,30 @@ func (server *server) handleClient(client *client) {
 			}
 			cmd := strings.ToUpper(input[:cmdLen])
 			switch {
+			case strings.Index(cmd, "PROXY ") == 0:
+				if firstCmd == false {
+					client.sendResponse(fmt.Sprintf("%d%s %s", response.ClassPermanentFailure, response.InvalidCommand, "PROXY must be the first command"))
+					break
+				}
+				proxyTokens := strings.Split(cmd, " ")
+				if len(proxyTokens) <= 1 {
+					client.sendResponse(fmt.Sprintf("%d%s %s", response.ClassPermanentFailure, response.InvalidCommandArguments, "Invalid PROXY arguments"))
+					break
+				}
+				proxyL4Proto := proxyTokens[1]
+				switch proxyL4Proto {
+				case "TCP4":
+					fallthrough
+				case "TCP6":
+					proxyL3Src := proxyTokens[2]
+					server.log().Debugf("Updating client IP from %s to %s", client.RemoteIP, proxyL3Src)
+					client.RemoteIP = proxyL3Src
+				case "UNKNOWN":
+				default:
+					client.sendResponse(fmt.Sprintf("%d%s Invalid PROXY protocol: %s", response.ClassPermanentFailure, response.InvalidCommandArguments, proxyL4Proto))
+				}
+				client.sendResponse("")
+
 			case strings.Index(cmd, "HELO") == 0:
 				client.Helo = strings.Trim(input[4:], " ")
 				client.resetTransaction()
@@ -530,6 +555,7 @@ func (server *server) handleClient(client *client) {
 					client.sendResponse(response.Canned.FailUnrecognizedCmd)
 				}
 			}
+			firstCmd = false
 
 		case ClientData:
 
diff --git a/tests/guerrilla_test.go b/tests/guerrilla_test.go
index 2ae85b9a..e6b63a5f 100644
--- a/tests/guerrilla_test.go
+++ b/tests/guerrilla_test.go
@@ -526,6 +526,78 @@ func TestRFC2821LimitDomain(t *testing.T) {
 	os.Truncate("./testlog", 0)
 }
 
+// Test support for Proxy Protocol
+func TestProxyProtocol(t *testing.T) {
+	if initErr != nil {
+		t.Error(initErr)
+		t.FailNow()
+	}
+	if startErrors := app.Start(); startErrors == nil {
+		conn, bufin, err := Connect(config.Servers[0], 20)
+		hostname := config.Servers[0].Hostname
+		if err != nil {
+			// handle error
+			t.Error(err.Error(), config.Servers[0].ListenInterface)
+			t.FailNow()
+		} else {
+			// Test PROXY header
+			response, err := Command(conn, bufin, "PROXY TCP4 1.1.1.1 2.2.2.2 12345 9876")
+			if err != nil {
+				t.Error("command failed", err.Error())
+			}
+			expected := ""
+			if strings.Index(response, expected) != 0 {
+				t.Error("Server did not respond with", expected, ", it said:"+response)
+			}
+			// Reset
+			response, err = Command(conn, bufin, "RSET")
+			if err != nil {
+				t.Error("command failed", err.Error())
+			}
+			expected = "250 2.1.0 OK"
+			if strings.Index(response, expected) != 0 {
+				t.Error("Server did not respond with", expected, ", it said:"+response)
+			}
+			// Start a new transaction
+			response, err = Command(conn, bufin, "HELO localtester")
+			if err != nil {
+				t.Error("command failed", err.Error())
+			}
+			expected = fmt.Sprintf("250 %s Hello", hostname)
+			if strings.Index(response, expected) != 0 {
+				t.Error("Server did not respond with", expected, ", it said:"+response)
+			}
+			// Send the PROXY header, but not as the first header
+			response, err = Command(conn, bufin, "PROXY TCP4 1.1.1.1 2.2.2.2 12345 9876")
+			if err != nil {
+				t.Error("command failed", err.Error())
+			}
+			expected = "5.5.1 PROXY must be the first command"
+			if strings.Index(response, expected) != 0 {
+				t.Error("Server did not respond with", expected, ", it said:"+response)
+			}
+
+			// be kind, QUIT. And we are sure that bufin does not contain fragments from the EHLO command.
+			response, err = Command(conn, bufin, "QUIT")
+			if err != nil {
+				t.Error("command failed", err.Error())
+			}
+			expected = "221 2.0.0 Bye"
+			if strings.Index(response, expected) != 0 {
+				t.Error("Server did not respond with", expected, ", it said:"+response)
+			}
+		}
+		conn.Close()
+		app.Shutdown()
+	} else {
+		if startErrors := app.Start(); startErrors != nil {
+			t.Error(startErrors)
+			app.Shutdown()
+			t.FailNow()
+		}
+	}
+}
+
 // Test several different inputs to MAIL FROM command
 func TestMailFromCmd(t *testing.T) {
 	if initErr != nil {