diff --git a/kernel/ethosu_mailbox.c b/kernel/ethosu_mailbox.c
index 4f7f5b7..5cc2465 100644
--- a/kernel/ethosu_mailbox.c
+++ b/kernel/ethosu_mailbox.c
@@ -28,6 +28,7 @@
 #include "ethosu_core_rpmsg.h"
 #include "ethosu_device.h"
 
+#include <linux/atomic.h>
 #include <linux/jiffies.h>
 #include <linux/resource.h>
 #include <linux/uio.h>
@@ -46,9 +47,81 @@
 #endif
 
 /****************************************************************************
+ * Defines
+ ****************************************************************************/
+
+#define MAILBOX_SEND_TIMEOUT_MS 15000
+
+/****************************************************************************
  * Functions
  ****************************************************************************/
 
+/**
+ * ethosu_send_locked() - Blocking mailbox message sender
+ *
+ * Context: Can sleep and must be called with the device mutex locked.
+ *
+ * Return: 0 on success, else error code.
+ */
+static int ethosu_send_locked(struct ethosu_mailbox *mbox,
+			      void *data,
+			      size_t length)
+{
+	DEFINE_WAIT_FUNC(wait, woken_wake_function);
+	struct device *dev = mbox->dev;
+	long timeout = msecs_to_jiffies(MAILBOX_SEND_TIMEOUT_MS);
+	bool try_send = !wq_has_sleeper(&mbox->send_queue);
+	int ret;
+
+	might_sleep();
+
+	/* Exclusive wait to only wake up one task at a time */
+	add_wait_queue_exclusive(&mbox->send_queue, &wait);
+	for (;;) {
+		/* Stop if the mailbox is closing down */
+		if (atomic_read(&mbox->done)) {
+			ret = -ENODEV;
+			break;
+		}
+
+		/* Attempt to send if queue is empty or task was woken up */
+		if (try_send) {
+			ret = rpmsg_trysend(mbox->ept, data, length);
+			if (ret != -ENOMEM)
+				break;
+		} else {
+			try_send = true;
+		}
+
+		/* Unlock device mutex while waiting to not block other tasks */
+		device_unlock(dev);
+		timeout = wait_woken(&wait, TASK_INTERRUPTIBLE, timeout);
+		device_lock(dev);
+
+		/* Stop if the wait was interrupted */
+		if (signal_pending(current)) {
+			ret = -EINTR;
+			break;
+		}
+
+		if (!timeout) {
+			ret = -ETIME;
+			break;
+		}
+	}
+
+	remove_wait_queue(&mbox->send_queue, &wait);
+
+	/*
+	 * If the message was sent successfully, there may be more TX buffers
+	 * available so wake up the next waiting task.
+	 */
+	if (!ret && wq_has_sleeper(&mbox->send_queue))
+		wake_up(&mbox->send_queue);
+
+	return ret;
+}
+
 static void ethosu_core_set_size(struct ethosu_buffer *buf,
 				 struct ethosu_core_buffer *cbuf)
 {
@@ -119,7 +192,7 @@
 		}
 	};
 
-	return rpmsg_send(mbox->ept, &rpmsg, sizeof(rpmsg.header));
+	return ethosu_send_locked(mbox, &rpmsg, sizeof(rpmsg.header));
 }
 
 int ethosu_mailbox_pong(struct ethosu_mailbox *mbox)
@@ -131,7 +204,7 @@
 		}
 	};
 
-	return rpmsg_send(mbox->ept, &rpmsg, sizeof(rpmsg.header));
+	return ethosu_send_locked(mbox, &rpmsg, sizeof(rpmsg.header));
 }
 
 int ethosu_mailbox_version_request(struct ethosu_mailbox *mbox,
@@ -147,7 +220,7 @@
 
 	msg->type = rpmsg.header.type;
 
-	return rpmsg_send(mbox->ept, &rpmsg, sizeof(rpmsg.header));
+	return ethosu_send_locked(mbox, &rpmsg, sizeof(rpmsg.header));
 }
 
 int ethosu_mailbox_capabilities_request(struct ethosu_mailbox *mbox,
@@ -163,7 +236,7 @@
 
 	msg->type = rpmsg.header.type;
 
-	return rpmsg_send(mbox->ept, &rpmsg, sizeof(rpmsg.header));
+	return ethosu_send_locked(mbox, &rpmsg, sizeof(rpmsg.header));
 }
 
 int ethosu_mailbox_inference(struct ethosu_mailbox *mbox,
@@ -218,8 +291,8 @@
 		inf_req->network.index = network_index;
 	}
 
-	return rpmsg_send(mbox->ept, &rpmsg,
-			  sizeof(rpmsg.header) + sizeof(rpmsg.inf_req));
+	return ethosu_send_locked(mbox, &rpmsg,
+				  sizeof(rpmsg.header) + sizeof(rpmsg.inf_req));
 }
 
 int ethosu_mailbox_network_info_request(struct ethosu_mailbox *mbox,
@@ -246,8 +319,9 @@
 		info_req->network.index = network_index;
 	}
 
-	return rpmsg_send(mbox->ept, &rpmsg,
-			  sizeof(rpmsg.header) + sizeof(rpmsg.net_info_req));
+	return ethosu_send_locked(mbox, &rpmsg,
+				  sizeof(rpmsg.header) +
+				  sizeof(rpmsg.net_info_req));
 }
 
 int ethosu_mailbox_cancel_inference(struct ethosu_mailbox *mbox,
@@ -268,8 +342,9 @@
 
 	msg->type = rpmsg.header.type;
 
-	return rpmsg_send(mbox->ept, &rpmsg,
-			  sizeof(rpmsg.header) + sizeof(rpmsg.cancel_req));
+	return ethosu_send_locked(mbox, &rpmsg,
+				  sizeof(rpmsg.header) +
+				  sizeof(rpmsg.cancel_req));
 }
 
 int ethosu_mailbox_init(struct ethosu_mailbox *mbox,
@@ -279,9 +354,13 @@
 	mbox->dev = dev;
 	mbox->ept = ept;
 	idr_init(&mbox->msg_idr);
+	init_waitqueue_head(&mbox->send_queue);
 
 	return 0;
 }
 
 void ethosu_mailbox_deinit(struct ethosu_mailbox *mbox)
-{}
+{
+	atomic_set(&mbox->done, 1);
+	wake_up_all(&mbox->send_queue);
+}