diff --git a/src/api/api_lib.c b/src/api/api_lib.c
index bee73f79..42edc57c 100644
--- a/src/api/api_lib.c
+++ b/src/api/api_lib.c
@@ -94,10 +94,14 @@
 #if LWIP_NETCONN_FULLDUPLEX
 #define NETCONN_RECVMBOX_WAITABLE(conn) (sys_mbox_valid(&(conn)->recvmbox) && (((conn)->flags & NETCONN_FLAG_MBOXINVALID) == 0))
 #define NETCONN_ACCEPTMBOX_WAITABLE(conn) (sys_mbox_valid(&(conn)->acceptmbox) && (((conn)->flags & (NETCONN_FLAG_MBOXCLOSED|NETCONN_FLAG_MBOXINVALID)) == 0))
-#else
+#define NETCONN_MBOX_WAITING_INC(conn) SYS_ARCH_INC(conn->mbox_threads_waiting, 1)
+#define NETCONN_MBOX_WAITING_DEC(conn) SYS_ARCH_INC(conn->mbox_threads_waiting, 1)
+#else /* LWIP_NETCONN_FULLDUPLEX */
 #define NETCONN_RECVMBOX_WAITABLE(conn)   sys_mbox_valid(&(conn)->recvmbox)
 #define NETCONN_ACCEPTMBOX_WAITABLE(conn) (sys_mbox_valid(&(conn)->acceptmbox) && (((conn)->flags & NETCONN_FLAG_MBOXCLOSED) == 0))
-#endif
+#define NETCONN_MBOX_WAITING_INC(conn)
+#define NETCONN_MBOX_WAITING_DEC(conn)
+#endif /* LWIP_NETCONN_FULLDUPLEX */
 
 static err_t netconn_close_shutdown(struct netconn *conn, u8_t how);
 
@@ -494,21 +498,35 @@ netconn_accept(struct netconn *conn, struct netconn **new_conn)
 
   API_MSG_VAR_ALLOC_ACCEPT(msg);
 
+  NETCONN_MBOX_WAITING_INC(conn);
   if (netconn_is_nonblocking(conn)) {
     if (sys_arch_mbox_tryfetch(&conn->acceptmbox, &accept_ptr) == SYS_ARCH_TIMEOUT) {
       API_MSG_VAR_FREE_ACCEPT(msg);
+      NETCONN_MBOX_WAITING_DEC(conn);
       return ERR_WOULDBLOCK;
     }
   } else {
 #if LWIP_SO_RCVTIMEO
     if (sys_arch_mbox_fetch(&conn->acceptmbox, &accept_ptr, conn->recv_timeout) == SYS_ARCH_TIMEOUT) {
       API_MSG_VAR_FREE_ACCEPT(msg);
+      NETCONN_MBOX_WAITING_DEC(conn);
       return ERR_TIMEOUT;
     }
 #else
     sys_arch_mbox_fetch(&conn->acceptmbox, &accept_ptr, 0);
 #endif /* LWIP_SO_RCVTIMEO*/
   }
+  NETCONN_MBOX_WAITING_DEC(conn);
+#if LWIP_NETCONN_FULLDUPLEX
+  if (conn->flags & NETCONN_FLAG_MBOXINVALID) {
+    if (lwip_netconn_is_deallocated_msg(accept_ptr)) {
+      /* the netconn has been closed from another thread */
+      API_MSG_VAR_FREE_ACCEPT(msg);
+      return ERR_CONN;
+    }
+  }
+#endif
+
   /* Register event with callback */
   API_EVENT(conn, NETCONN_EVT_RCVMINUS, 0);
 
@@ -576,10 +594,13 @@ netconn_recv_data(struct netconn *conn, void **new_buf, u8_t apiflags)
     return ERR_CONN;
   }
 
+  NETCONN_MBOX_WAITING_INC(conn);
   if (netconn_is_nonblocking(conn) || (apiflags & NETCONN_DONTBLOCK) ||
       (conn->flags & NETCONN_FLAG_MBOXCLOSED) || (conn->pending_err != ERR_OK)) {
     if (sys_arch_mbox_tryfetch(&conn->recvmbox, &buf) == SYS_ARCH_TIMEOUT) {
-      err_t err = netconn_err(conn);
+      err_t err;
+      NETCONN_MBOX_WAITING_DEC(conn);
+      err = netconn_err(conn);
       if (err != ERR_OK) {
         /* return pending error */
         return err;
@@ -592,12 +613,23 @@ netconn_recv_data(struct netconn *conn, void **new_buf, u8_t apiflags)
   } else {
 #if LWIP_SO_RCVTIMEO
     if (sys_arch_mbox_fetch(&conn->recvmbox, &buf, conn->recv_timeout) == SYS_ARCH_TIMEOUT) {
+      NETCONN_MBOX_WAITING_DEC(conn);
       return ERR_TIMEOUT;
     }
 #else
     sys_arch_mbox_fetch(&conn->recvmbox, &buf, 0);
 #endif /* LWIP_SO_RCVTIMEO*/
   }
+  NETCONN_MBOX_WAITING_DEC(conn);
+#if LWIP_NETCONN_FULLDUPLEX
+  if (conn->flags & NETCONN_FLAG_MBOXINVALID) {
+    if (lwip_netconn_is_deallocated_msg(buf)) {
+      /* the netconn has been closed from another thread */
+      API_MSG_VAR_FREE_ACCEPT(msg);
+      return ERR_CONN;
+    }
+  }
+#endif
 
 #if LWIP_TCP
 #if (LWIP_UDP || LWIP_RAW)
diff --git a/src/api/api_msg.c b/src/api/api_msg.c
index 254b7559..614ca641 100644
--- a/src/api/api_msg.c
+++ b/src/api/api_msg.c
@@ -92,6 +92,19 @@ static void netconn_drain(struct netconn *conn);
 #define TCPIP_APIMSG_ACK(m)   do { sys_sem_signal(LWIP_API_MSG_SEM(m)); } while(0)
 #endif /* LWIP_TCPIP_CORE_LOCKING */
 
+#if LWIP_NETCONN_FULLDUPLEX
+const u8_t netconn_deleted = 0;
+
+int
+lwip_netconn_is_deallocated_msg(void *msg)
+{
+  if (msg == &netconn_deleted) {
+    return 1;
+  }
+  return 0;
+}
+#endif /* LWIP_NETCONN_FULLDUPLEX */
+
 #if LWIP_TCP
 const u8_t netconn_aborted = 0;
 const u8_t netconn_reset = 0;
@@ -823,16 +836,21 @@ netconn_drain(struct netconn *conn)
   /* Delete and drain the recvmbox. */
   if (sys_mbox_valid(&conn->recvmbox)) {
     while (sys_mbox_tryfetch(&conn->recvmbox, &mem) != SYS_MBOX_EMPTY) {
-#if LWIP_TCP
-      if (NETCONNTYPE_GROUP(conn->type) == NETCONN_TCP) {
-        err_t err;
-        if (!lwip_netconn_is_err_msg(mem, &err)) {
-          pbuf_free((struct pbuf *)mem);
-        }
-      } else
-#endif /* LWIP_TCP */
+#if LWIP_NETCONN_FULLDUPLEX
+      if (!lwip_netconn_is_deallocated_msg(mem))
+#endif /* LWIP_NETCONN_FULLDUPLEX */
       {
-        netbuf_delete((struct netbuf *)mem);
+#if LWIP_TCP
+        if (NETCONNTYPE_GROUP(conn->type) == NETCONN_TCP) {
+          err_t err;
+          if (!lwip_netconn_is_err_msg(mem, &err)) {
+            pbuf_free((struct pbuf *)mem);
+          }
+        } else
+#endif /* LWIP_TCP */
+        {
+          netbuf_delete((struct netbuf *)mem);
+        }
       }
     }
     sys_mbox_free(&conn->recvmbox);
@@ -843,18 +861,23 @@ netconn_drain(struct netconn *conn)
 #if LWIP_TCP
   if (sys_mbox_valid(&conn->acceptmbox)) {
     while (sys_mbox_tryfetch(&conn->acceptmbox, &mem) != SYS_MBOX_EMPTY) {
-      err_t err;
-      if (!lwip_netconn_is_err_msg(mem, &err)) {
-        struct netconn *newconn = (struct netconn *)mem;
-        /* Only tcp pcbs have an acceptmbox, so no need to check conn->type */
-        /* pcb might be set to NULL already by err_tcp() */
-        /* drain recvmbox */
-        netconn_drain(newconn);
-        if (newconn->pcb.tcp != NULL) {
-          tcp_abort(newconn->pcb.tcp);
-          newconn->pcb.tcp = NULL;
+#if LWIP_NETCONN_FULLDUPLEX
+      if (!lwip_netconn_is_deallocated_msg(mem))
+#endif /* LWIP_NETCONN_FULLDUPLEX */
+      {
+        err_t err;
+        if (!lwip_netconn_is_err_msg(mem, &err)) {
+          struct netconn *newconn = (struct netconn *)mem;
+          /* Only tcp pcbs have an acceptmbox, so no need to check conn->type */
+          /* pcb might be set to NULL already by err_tcp() */
+          /* drain recvmbox */
+          netconn_drain(newconn);
+          if (newconn->pcb.tcp != NULL) {
+            tcp_abort(newconn->pcb.tcp);
+            newconn->pcb.tcp = NULL;
+          }
+          netconn_free(newconn);
         }
-        netconn_free(newconn);
       }
     }
     sys_mbox_free(&conn->acceptmbox);
@@ -867,8 +890,20 @@ netconn_drain(struct netconn *conn)
 static void
 netconn_mark_mbox_invalid(struct netconn *conn)
 {
+  int i, num_waiting;
+  void *msg = LWIP_CONST_CAST(void *, &netconn_deleted);
+
   /* Prevent new calls/threads from reading from the mbox */
   conn->flags |= NETCONN_FLAG_MBOXINVALID;
+
+  SYS_ARCH_LOCKED(num_waiting = conn->mbox_threads_waiting);
+  for (i = 0; i < num_waiting; i++) {
+    if (sys_mbox_valid_val(conn->recvmbox)) {
+      sys_mbox_trypost(&conn->recvmbox, msg);
+    } else {
+      sys_mbox_trypost(&conn->acceptmbox, msg);
+    }
+  }
 }
 #endif /* LWIP_NETCONN_FULLDUPLEX */
 
diff --git a/src/include/lwip/api.h b/src/include/lwip/api.h
index fb1e05a7..f8426a7d 100644
--- a/src/include/lwip/api.h
+++ b/src/include/lwip/api.h
@@ -241,6 +241,11 @@ struct netconn {
       by the application thread */
   sys_mbox_t acceptmbox;
 #endif /* LWIP_TCP */
+#if LWIP_NETCONN_FULLDUPLEX
+  /** number of threads waiting on an mbox. This is required to unblock
+      all threads when closing while threads are waiting. */
+  int mbox_threads_waiting;
+#endif
   /** only used for socket layer */
 #if LWIP_SOCKET
   int socket;
diff --git a/src/include/lwip/priv/api_msg.h b/src/include/lwip/priv/api_msg.h
index 9dfd088f..9e8ffc9e 100644
--- a/src/include/lwip/priv/api_msg.h
+++ b/src/include/lwip/priv/api_msg.h
@@ -187,6 +187,9 @@ struct dns_api_msg {
 };
 #endif /* LWIP_DNS */
 
+#if LWIP_NETCONN_FULLDUPLEX
+int lwip_netconn_is_deallocated_msg(void *msg);
+#endif
 int lwip_netconn_is_err_msg(void *msg, err_t *err);
 void lwip_netconn_do_newconn         (void *m);
 void lwip_netconn_do_delconn         (void *m);