diff --git a/src/java/davmail/caldav/CaldavConnection.java b/src/java/davmail/caldav/CaldavConnection.java
index c2c21bf2..391f872f 100644
--- a/src/java/davmail/caldav/CaldavConnection.java
+++ b/src/java/davmail/caldav/CaldavConnection.java
@@ -220,7 +220,8 @@ public class CaldavConnection extends AbstractConnection {
                 // only current user for now
                 && session.getEmail().equalsIgnoreCase(paths[2])) {
             String etag = headers.get("if-match");
-            int status = session.createOrUpdateEvent(paths[4], body, etag);
+            String noneMatch = headers.get("if-none-match"); 
+            int status = session.createOrUpdateEvent(paths[4], body, etag, noneMatch);
             sendHttpResponse(status, true);
 
         } else if ("DELETE".equals(command) && "users".equals(paths[1]) && paths.length == 5 && "calendar".equals(paths[3])
diff --git a/src/java/davmail/exchange/ExchangeSession.java b/src/java/davmail/exchange/ExchangeSession.java
index 39a70b89..d18dfc3a 100644
--- a/src/java/davmail/exchange/ExchangeSession.java
+++ b/src/java/davmail/exchange/ExchangeSession.java
@@ -1027,7 +1027,7 @@ public class ExchangeSession {
         return line.substring(0, keyIndex) + ";VALUE=DATE:" + line.substring(valueIndex + 1, valueEndIndex);
     }
 
-    public int createOrUpdateEvent(String path, String icsBody, String etag) throws IOException {
+    public int createOrUpdateEvent(String path, String icsBody, String etag, String noneMatch) throws IOException {
         String messageUrl = URIUtil.encodePathQuery(calendarUrl + "/" + URIUtil.decode(path));
         String uid = path.substring(0, path.lastIndexOf("."));
         PutMethod putmethod = new PutMethod(messageUrl);
@@ -1036,6 +1036,9 @@ public class ExchangeSession {
         if (etag != null) {
             putmethod.setRequestHeader("If-Match", etag);
         }
+        if (noneMatch != null) {
+            putmethod.setRequestHeader("If-None-Match", noneMatch);
+        }
         putmethod.setRequestHeader("Content-Type", "message/rfc822");
         StringBuilder body = new StringBuilder();
         body.append("Content-Transfer-Encoding: 7bit\n" +