diff --git a/CHANGELOG b/CHANGELOG
index 8e6da0f3..097d85c2 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -19,6 +19,19 @@ HISTORY
 
   ++ New features:
 
+  2008-01-05 Frédéric Bernon
+  * sys_arch.txt, api.h, api_lib.c, api_msg.h, api_msg.c, tcpip.c, sys.h, opt.h:
+    Introduce changes for task #7490 "Add return value to sys_mbox_post" with some
+    modifications in the sys_mbox api: sys_mbox_new take a "size" parameters which
+    indicate the number of pointers query by the mailbox. There is three defines
+    in opt.h to indicate sizes for tcpip::mbox, netconn::recvmbox, and for the 
+    netconn::acceptmbox. Port maintainers, you can decide to just add this new 
+    parameter in your implementation, but to ignore it to keep the previous behavior.
+    The new sys_mbox_trypost function return a value to know if the mailbox is
+    full or if the message is posted. Take a look to sys_arch.txt for more details.
+    This new function is used in tcpip_input (so, can be called in an interrupt
+    context since the function is not blocking), and in recv_udp and recv_raw.
+
   2008-01-04 Frédéric Bernon, Simon Goldschmidt, Jonathan Larmour
   * rawapi.txt, api.h, api_lib.c, api_msg.h, api_msg.c, sockets.c, tcp.h, tcp.c,
     tcp_in.c, init.c, opt.h: rename backlog options with TCP_ prefix, limit the
diff --git a/doc/sys_arch.txt b/doc/sys_arch.txt
index 77a52424..970f19f6 100644
--- a/doc/sys_arch.txt
+++ b/doc/sys_arch.txt
@@ -69,9 +69,12 @@ The following functions must be implemented by the sys_arch:
   Notice that lwIP implements a function with a similar name,
   sys_sem_wait(), that uses the sys_arch_sem_wait() function.
 
-- sys_mbox_t sys_mbox_new(void)
+- sys_mbox_t sys_mbox_new(int size)
 
-  Creates an empty mailbox.
+  Creates an empty mailbox for maximum "size" elements. Elements stored
+  in mailboxes are pointers. You have to define macros "_MBOX_SIZE"
+  in your lwipopts.h, or ignore this parameter in your implementation
+  and use a default size.
 
 - void sys_mbox_free(sys_mbox_t mbox)
 
@@ -81,7 +84,13 @@ The following functions must be implemented by the sys_arch:
 
 - void sys_mbox_post(sys_mbox_t mbox, void *msg)
 
-  Posts the "msg" to the mailbox.
+  Posts the "msg" to the mailbox. This function have to block until
+  the "msg" is really posted.
+
+- err_t sys_mbox_trypost(sys_mbox_t mbox, void *msg)
+
+  Try to post the "msg" to the mailbox. Returns ERR_MEM if this one
+  is full, else, ERR_OK if the "msg" is posted.
 
 - u32_t sys_arch_mbox_fetch(sys_mbox_t mbox, void **msg, u32_t timeout)
 
diff --git a/src/api/api_lib.c b/src/api/api_lib.c
index f5a16e8f..77f715b1 100644
--- a/src/api/api_lib.c
+++ b/src/api/api_lib.c
@@ -65,8 +65,7 @@
  *         NULL on memory error
  */
 struct netconn*
-netconn_new_with_proto_and_callback(enum netconn_type t, u8_t proto,
-          void (*callback)(struct netconn *, enum netconn_evt, u16_t len))
+netconn_new_with_proto_and_callback(enum netconn_type t, u8_t proto, netconn_callback callback)
 {
   struct netconn *conn;
   struct api_msg msg;
diff --git a/src/api/api_msg.c b/src/api/api_msg.c
index 4add74b9..fa762fdc 100644
--- a/src/api/api_msg.c
+++ b/src/api/api_msg.c
@@ -98,7 +98,9 @@ recv_raw(void *arg, struct raw_pcb *pcb, struct pbuf *p,
     SYS_ARCH_INC(conn->recv_avail, p->tot_len);
     /* Register event with callback */
     API_EVENT(conn, NETCONN_EVT_RCVPLUS, p->tot_len);
-    sys_mbox_post(conn->recvmbox, buf);
+    if (sys_mbox_trypost(conn->recvmbox, buf) != ERR_OK) {
+      netbuf_delete(buf);
+    }
   }
 
   return 0; /* do not eat the packet */
@@ -153,7 +155,10 @@ recv_udp(void *arg, struct udp_pcb *pcb, struct pbuf *p,
   SYS_ARCH_INC(conn->recv_avail, p->tot_len);
   /* Register event with callback */
   API_EVENT(conn, NETCONN_EVT_RCVPLUS, p->tot_len);
-  sys_mbox_post(conn->recvmbox, buf);
+  if (sys_mbox_trypost(conn->recvmbox, buf) != ERR_OK) {
+    netbuf_delete(buf);
+    return;
+  }
 }
 #endif /* LWIP_UDP */
 
@@ -348,7 +353,11 @@ accept_function(void *arg, struct tcp_pcb *newpcb, err_t err)
   /* Register event with callback */
   API_EVENT(conn, NETCONN_EVT_RCVPLUS, 0);
 
-  sys_mbox_post(conn->acceptmbox, newconn);
+  if (sys_mbox_trypost(conn->acceptmbox, newconn) != ERR_OK) {
+    /** @todo call here a "netconn_free" */
+    LWIP_ASSERT("accept_function: not yet implemented!", 0);
+    return ERR_MEM;
+  }
   return ERR_OK;
 }
 #endif /* LWIP_TCP */
@@ -446,8 +455,7 @@ do_newconn(struct api_msg_msg *msg)
  *         NULL on memory error
  */
 struct netconn*
-netconn_alloc(enum netconn_type t,
-          void (*callback)(struct netconn *, enum netconn_evt, u16_t len))
+netconn_alloc(enum netconn_type t, netconn_callback callback)
 {
   struct netconn *conn;
 
@@ -460,11 +468,11 @@ netconn_alloc(enum netconn_type t,
   conn->type = t;
   conn->pcb.tcp = NULL;
 
-  if ((conn->mbox = sys_mbox_new()) == SYS_MBOX_NULL) {
+  if ((conn->mbox = sys_mbox_new(1)) == SYS_MBOX_NULL) {
     memp_free(MEMP_NETCONN, conn);
     return NULL;
   }
-  if ((conn->recvmbox = sys_mbox_new()) == SYS_MBOX_NULL) {
+  if ((conn->recvmbox = sys_mbox_new(DEFAULT_RECVMBOX_SIZE)) == SYS_MBOX_NULL) {
     sys_mbox_free(conn->mbox);
     memp_free(MEMP_NETCONN, conn);
     return NULL;
@@ -732,7 +740,7 @@ do_listen(struct api_msg_msg *msg)
               msg->conn->recvmbox = NULL;
             }
             if (msg->conn->acceptmbox == SYS_MBOX_NULL) {
-              if ((msg->conn->acceptmbox = sys_mbox_new()) == SYS_MBOX_NULL) {
+              if ((msg->conn->acceptmbox = sys_mbox_new(DEFAULT_ACCEPTMBOX_SIZE)) == SYS_MBOX_NULL) {
                 msg->conn->err = ERR_MEM;
               }
             }
diff --git a/src/api/tcpip.c b/src/api/tcpip.c
index 65faaa88..e68c805f 100644
--- a/src/api/tcpip.c
+++ b/src/api/tcpip.c
@@ -332,7 +332,10 @@ tcpip_input(struct pbuf *p, struct netif *inp)
     msg->type = TCPIP_MSG_INPKT;
     msg->msg.inp.p = p;
     msg->msg.inp.netif = inp;
-    sys_mbox_post(mbox, msg);
+    if (sys_mbox_trypost(mbox, msg) != ERR_OK) {
+      memp_free(MEMP_TCPIP_MSG_INPKT, msg);
+      return ERR_MEM;
+    }
     return ERR_OK;
   }
   return ERR_VAL;
@@ -499,7 +502,7 @@ tcpip_init(void (* initfunc)(void *), void *arg)
 
   tcpip_init_done = initfunc;
   tcpip_init_done_arg = arg;
-  mbox = sys_mbox_new();
+  mbox = sys_mbox_new(TCPIP_MBOX_SIZE);
 #if LWIP_TCPIP_CORE_LOCKING
   lock_tcpip_core = sys_sem_new(1);
 #endif /* LWIP_TCPIP_CORE_LOCKING */
diff --git a/src/include/lwip/api.h b/src/include/lwip/api.h
index 0195df88..4e8a1b4b 100644
--- a/src/include/lwip/api.h
+++ b/src/include/lwip/api.h
@@ -98,6 +98,10 @@ struct ip_pcb;
 struct tcp_pcb;
 struct udp_pcb;
 struct raw_pcb;
+struct netconn;
+
+/** A callback prototype to inform about events for a netconn */
+typedef void (* netconn_callback)(struct netconn *, enum netconn_evt, u16_t len);
 
 /** A netconn descriptor */
 struct netconn {
@@ -148,7 +152,7 @@ struct netconn {
   u8_t write_delayed;
 #endif /* LWIP_TCPIP_CORE_LOCKING */
   /** A callback function that is informed about events for this netconn */
-  void (* callback)(struct netconn *, enum netconn_evt, u16_t len);
+  netconn_callback callback;
 };
 
 /* Register an Network connection event */
@@ -161,7 +165,7 @@ struct netconn {
 #define netconn_new_with_callback(t, c) netconn_new_with_proto_and_callback(t, 0, c)
 struct
 netconn *netconn_new_with_proto_and_callback(enum netconn_type t, u8_t proto,
-                                   void (*callback)(struct netconn *, enum netconn_evt, u16_t len));
+                                   netconn_callback callback);
 err_t             netconn_delete  (struct netconn *conn);
 enum netconn_type netconn_type    (struct netconn *conn);
 
diff --git a/src/include/lwip/api_msg.h b/src/include/lwip/api_msg.h
index 1691a889..e6cd1590 100644
--- a/src/include/lwip/api_msg.h
+++ b/src/include/lwip/api_msg.h
@@ -148,9 +148,7 @@ void do_join_leave_group( struct api_msg_msg *msg);
 void do_gethostbyname(void *arg);
 #endif /* LWIP_DNS */
 
-struct netconn* netconn_alloc(
-  enum netconn_type t,
-  void (*callback)(struct netconn *, enum netconn_evt, u16_t len));
+struct netconn* netconn_alloc(enum netconn_type t, netconn_callback callback);
 
 #ifdef __cplusplus
 }
diff --git a/src/include/lwip/opt.h b/src/include/lwip/opt.h
index 9ec3a426..461e6f35 100644
--- a/src/include/lwip/opt.h
+++ b/src/include/lwip/opt.h
@@ -874,6 +874,15 @@
 #define TCPIP_THREAD_PRIO               1
 #endif
 
+/**
+ * TCPIP_MBOX_SIZE: The mailbox size for the tcpip thread messages
+ * The queue size value itself is platform-dependent, but is passed to
+ * sys_mbox_new() when tcpip_init is called.
+ */
+#ifndef TCPIP_MBOX_SIZE
+#define TCPIP_MBOX_SIZE                 0
+#endif
+
 /**
  * SLIPIF_THREAD_NAME: The name assigned to the slipif_loop thread.
  */
@@ -949,6 +958,24 @@
 #define DEFAULT_THREAD_PRIO             1
 #endif
 
+/**
+ * DEFAULT_RECVMBOX_SIZE: The mailbox size for the incoming packets.
+ * The queue size value itself is platform-dependent, but is passed to
+ * sys_mbox_new() when the recvmbox is created.
+ */
+#ifndef DEFAULT_RECVMBOX_SIZE
+#define DEFAULT_RECVMBOX_SIZE           0
+#endif
+
+/**
+ * DEFAULT_ACCEPTMBOX_SIZE: The mailbox size for the incoming connections.
+ * The queue size value itself is platform-dependent, but is passed to
+ * sys_mbox_new() when the acceptmbox is created.
+ */
+#ifndef DEFAULT_ACCEPTMBOX_SIZE
+#define DEFAULT_ACCEPTMBOX_SIZE         0
+#endif
+
 /*
    ----------------------------------------------
    ---------- Sequential layer options ----------
diff --git a/src/include/lwip/sys.h b/src/include/lwip/sys.h
index 6927886e..ce73beac 100644
--- a/src/include/lwip/sys.h
+++ b/src/include/lwip/sys.h
@@ -56,9 +56,11 @@ struct sys_timeo {u8_t dummy;};
 #define sys_sem_wait_timeout(s,t)
 #define sys_arch_sem_wait(s,t)
 #define sys_sem_free(s)
-#define sys_mbox_new() 0
+#define sys_mbox_new(s) 0
 #define sys_mbox_fetch(m,d)
+#define sys_mbox_tryfetch(m,d)
 #define sys_mbox_post(m,d)
+#define sys_mbox_trypost(m,d)
 #define sys_mbox_free(m)
 
 #define sys_thread_new(n,t,a,s,p)
@@ -73,6 +75,7 @@ struct sys_timeo {u8_t dummy;};
  */
 #define SYS_MBOX_EMPTY SYS_ARCH_TIMEOUT 
 
+#include "lwip/err.h"
 #include "arch/sys_arch.h"
 
 typedef void (* sys_timeout_handler)(void *arg);
@@ -121,8 +124,9 @@ u32_t sys_jiffies(void); /* since power up. */
 #endif
 
 /* Mailbox functions. */
-sys_mbox_t sys_mbox_new(void);
+sys_mbox_t sys_mbox_new(int size);
 void sys_mbox_post(sys_mbox_t mbox, void *msg);
+err_t sys_mbox_trypost(sys_mbox_t mbox, void *msg);
 u32_t sys_arch_mbox_fetch(sys_mbox_t mbox, void **msg, u32_t timeout);
 #ifndef sys_arch_mbox_tryfetch /* Allow port to override with a macro */
 u32_t sys_arch_mbox_tryfetch(sys_mbox_t mbox, void **msg);