diff --git a/src/net/isds_const.h b/src/net/isds_const.h
index 4f8e12663c51d3fa0a9bdc9f8b9d992915943359..14c2f59e71e6b6530d7f97c015dfc613fec1de51 100644
--- a/src/net/isds_const.h
+++ b/src/net/isds_const.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2014-2017 CZ.NIC
+ * Copyright (C) 2014-2018 CZ.NIC
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -8,7 +8,7 @@
  *
  * This program is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
@@ -21,8 +21,7 @@
  * the two.
  */
 
-#ifndef _ISDS_CONST_H_
-#define _ISDS_CONST_H_
+#pragma once
 
 /* HTTP request timeout, default 5 minutes */
 #define ISDS_TIMEOUT 300000
@@ -110,8 +109,14 @@
 #define DOWNLOAD_ALL_MESSAGES 2046
 
 /*!
- * @brief Maximum attachment size in kB for message sending (ISDS 20 MB).
+ * @brief Maximum attachment size in MB for message sending (ISDS 20 MB).
  */
-#define SEND_ATTACH_SIZE_LIMIT_KB 20000
+#define SEND_ATTACH_SIZE_LIMIT_MB 20
 
-#endif // _ISDS_CONST_H_
+/*!
+ * @brief Maximum attachment size in MB for message sending (ISDS OVM 50 MB).
+ *
+ * @note New ISDS message size limit is 50MB because some
+ *       OVM databoxes can receive message up to 50MB.
+ */
+#define SEND_OVM_ATTACH_SIZE_LIMIT_MB 50
diff --git a/src/net/isds_wrapper.cpp b/src/net/isds_wrapper.cpp
index 940d51f1787f11ee3105924a390c98bddb3288cf..05ae05bf54c4c18718abd8fc2a38904af2a1a195 100644
--- a/src/net/isds_wrapper.cpp
+++ b/src/net/isds_wrapper.cpp
@@ -8,7 +8,7 @@
  *
  * This program is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
@@ -712,17 +712,33 @@ void IsdsWrapper::sendMessage(const QString &userName, qint64 dmID,
 
 	envelope.setDmAttachmentSize(totalAttachmentSizeKBs);
 
-	/* Test if total attachment size is above the limit */
-	if (totalAttachmentSizeKBs > SEND_ATTACH_SIZE_LIMIT_KB) {
+	/* Get total attachment in MB for error dialogue */
+	int totalAttachmentSizeMB = totalAttachmentSizeKBs / 1024;
+
+	/* Test if total attachment size is above the limit 50MB */
+	if (totalAttachmentSizeMB > SEND_OVM_ATTACH_SIZE_LIMIT_MB) {
 		Dialogues::errorMessage(Dialogues::CRITICAL,
-		    tr("%1: Error sending message!").arg(userName),
+		    tr("%1: Message problem").arg(userName),
 		    tr("Total attachment size exceeds the limit!"),
-		    tr("Current total attachment size is %1 kB. ISDS limits its size to %2 kB. Reduce the total attachment size and try sending the message again.")
-		    .arg(totalAttachmentSizeKBs).arg(SEND_ATTACH_SIZE_LIMIT_KB)
+		    tr("Total attachment size is %1 MB. ISDS limits its size to %2 MB. Reduce the total attachment size and try sending the message again.")
+		    .arg(totalAttachmentSizeMB).arg(SEND_OVM_ATTACH_SIZE_LIMIT_MB)
 		);
 		return;
 	}
 
+	/* Test if total attachment size is larger then 20 MB */
+	if (totalAttachmentSizeMB >= SEND_ATTACH_SIZE_LIMIT_MB) {
+		int msgResponse = Dialogues::message(Dialogues::QUESTION,
+		    tr("%1: Message problem").arg(userName),
+		    tr("Total size of attachments exceeds %1 MB. Most of the data boxes cannot receive messages larger than %1 MB. However, some OVM data boxes can receive message up to %2 MB.")
+		    .arg(SEND_ATTACH_SIZE_LIMIT_MB).arg(SEND_OVM_ATTACH_SIZE_LIMIT_MB),
+		    tr("Do you really want to send the large message?"),
+		    Dialogues::NO | Dialogues::YES, Dialogues::NO);
+		if (msgResponse == Dialogues::NO) {
+			return;
+		}
+	}
+
 	const QList<DataboxModelEntry> dbList = databoxModel->allEntries();
 
 	/* List of unique identifiers. */