diff --git a/src/java/davmail/exchange/ews/EwsExchangeSession.java b/src/java/davmail/exchange/ews/EwsExchangeSession.java
index 06896e3c..8fe556d0 100644
--- a/src/java/davmail/exchange/ews/EwsExchangeSession.java
+++ b/src/java/davmail/exchange/ews/EwsExchangeSession.java
@@ -2450,8 +2450,8 @@ public class EwsExchangeSession extends ExchangeSession {
      */
     protected static boolean isItemId(String itemName) {
         return itemName.length() >= 152
-                // item name is base64
-                && itemName.matches("^([A-Za-z0-9+/]{4})*([A-Za-z0-9+/]{4}|[A-Za-z0-9+/]{3}=|[A-Za-z0-9+/]{2}==)$");
+                // item name is base64url
+                && itemName.matches("^([A-Za-z0-9-_]{4})*([A-Za-z0-9-_]{4}|[A-Za-z0-9-_]{3}=|[A-Za-z0-9-_]{2}==)$");
     }