Fix xclient allowproxy.

backend.go

@@ -148,7 +148,7 @@ type ProxySession interface {
 	// This is similar to ProxyBackend.AllowProxy but called instead of it if
 	// Session is already created for the user. Session.Logout will be
 	// called if this function returns true.
-	AllowProxy(asserted ConnectionState, sid string) bool
+	AllowProxy(asserted ConnectionState) bool
 }
 
 // LMTPSession is an add-on interface for Session. It can be implemented by

conn.go

@@ -1279,20 +1279,20 @@ func (c *Conn) handleXclient(arg string) {
 	c.locker.Lock()
 	defer c.locker.Unlock()
 
+	sessCheckAllowProxy := false
 	if c.session != nil {
-		// We safely check ProxyBackend before and require backends to implement both or none.
-		se := c.session.(ProxySession)
-
-		if !se.AllowProxy(*connState, c.sid) {
-			c.server.Logger.Infof("smtp/server sid=%s reject: addr=%s xclient-addr=%s not permitted",
-				c.sid, c.conn.RemoteAddr().String(), connState.RemoteAddr.String())
-			c.WriteResponse(550, EnhancedCode{5, 7, 0}, "XCLIENT not permitted")
-			return
+		se, ok := c.session.(ProxySession)
+		if ok {
+			sessCheckAllowProxy = true
+			if !se.AllowProxy(*connState) {
+				c.server.Logger.Infof("smtp/server sid=%s reject: addr=%s xclient-addr=%s not permitted",
+					c.sid, c.conn.RemoteAddr().String(), connState.RemoteAddr.String())
+				c.WriteResponse(550, EnhancedCode{5, 7, 0}, "XCLIENT not permitted")
+				return
+			}
 		}
-
-		c.session.Logout()
-		c.session = nil
-	} else {
+	}
+	if !sessCheckAllowProxy {
 		if !be.AllowProxy(c.State(), *connState, c.sid) {
 			c.server.Logger.Infof("smtp/server sid=%s reject: addr=%s xclient-addr=%s not permitted",
 				c.sid, c.conn.RemoteAddr().String(), connState.RemoteAddr.String())