diff --git a/src/java/davmail/exchange/dav/DavExchangeSession.java b/src/java/davmail/exchange/dav/DavExchangeSession.java
index 3d156f13..1daade75 100644
--- a/src/java/davmail/exchange/dav/DavExchangeSession.java
+++ b/src/java/davmail/exchange/dav/DavExchangeSession.java
@@ -640,6 +640,9 @@ public class DavExchangeSession extends ExchangeSession {
                 buffer.append(alias);
                 if (alias.indexOf('@') < 0) {
                     buffer.append('@');
+                    if (hostName == null) {
+                        hostName = "mail.unknown.com";
+                    }
                     int dotIndex = hostName.indexOf('.');
                     if (dotIndex >= 0) {
                         buffer.append(hostName.substring(dotIndex + 1));