diff --git a/src/java/davmail/http/DavGatewayX509TrustManager.java b/src/java/davmail/http/DavGatewayX509TrustManager.java
new file mode 100644
index 00000000..b44a9aeb
--- /dev/null
+++ b/src/java/davmail/http/DavGatewayX509TrustManager.java
@@ -0,0 +1,97 @@
+package davmail.http;
+
+import davmail.Settings;
+import davmail.tray.DavGatewayTray;
+import davmail.ui.AcceptCertificateDialog;
+
+import javax.net.ssl.TrustManager;
+import javax.net.ssl.TrustManagerFactory;
+import javax.net.ssl.X509TrustManager;
+import java.awt.*;
+import java.security.KeyStore;
+import java.security.KeyStoreException;
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+import java.security.cert.CertificateEncodingException;
+import java.security.cert.CertificateException;
+import java.security.cert.X509Certificate;
+
+/**
+ * Custom Trust Manager, let user accept or deny.
+ */
+public class DavGatewayX509TrustManager implements X509TrustManager {
+    private X509TrustManager standardTrustManager = null;
+
+    public DavGatewayX509TrustManager() throws NoSuchAlgorithmException, KeyStoreException {
+        super();
+        TrustManagerFactory factory = TrustManagerFactory.getInstance(TrustManagerFactory.getDefaultAlgorithm());
+        factory.init((KeyStore) null);
+        TrustManager[] trustmanagers = factory.getTrustManagers();
+        if (trustmanagers.length == 0) {
+            throw new NoSuchAlgorithmException("No trust manager found");
+        }
+        this.standardTrustManager = (X509TrustManager) trustmanagers[0];
+    }
+
+    public void checkServerTrusted(X509Certificate[] x509Certificates, String authType) throws CertificateException {
+        try {
+            // first try standard Trust Manager
+            this.standardTrustManager.checkServerTrusted(x509Certificates, authType);
+        } catch (CertificateException e) {
+            if ((x509Certificates != null) && (x509Certificates.length > 0) && !GraphicsEnvironment.isHeadless()) {
+                userCheckServerTrusted(x509Certificates);
+            } else {
+                throw e;
+            }
+
+        }
+    }
+
+    public void checkClientTrusted(X509Certificate[] x509Certificates, String authType) throws CertificateException {
+        this.standardTrustManager.checkClientTrusted(x509Certificates, authType);
+    }
+
+    public java.security.cert.X509Certificate[] getAcceptedIssuers() {
+        return this.standardTrustManager.getAcceptedIssuers();
+    }
+
+    protected void userCheckServerTrusted(final X509Certificate[] x509Certificates) throws CertificateException {
+        try {
+            String acceptedCertificateHash = Settings.getProperty("davmail.server.certificate.hash");
+            String certificateHash = getFormattedHash(x509Certificates[0]);
+            // if user already accepted a certificate,
+            if (acceptedCertificateHash != null && acceptedCertificateHash.length() > 0
+                    && acceptedCertificateHash.equals(certificateHash)) {
+                DavGatewayTray.debug("Found permanently accepted cerfificate, hash " + acceptedCertificateHash);
+            } else {
+
+                if (!AcceptCertificateDialog.isCertificateTrusted(x509Certificates[0])) {
+                    throw new CertificateException("User rejected certificate");
+                }
+                // certificate accepted, store in settings
+                Settings.saveProperty("davmail.server.certificate.hash", certificateHash);
+            }
+        } catch (NoSuchAlgorithmException nsa) {
+            throw new CertificateException(nsa);
+        }
+    }
+
+    public static String getFormattedHash(X509Certificate certificate) throws NoSuchAlgorithmException, CertificateEncodingException {
+        String sha1Hash;
+        MessageDigest md = MessageDigest.getInstance("SHA1");
+        byte[] digest = md.digest(certificate.getEncoded());
+        sha1Hash = formatHash(digest);
+        return sha1Hash;
+    }
+
+    public static String formatHash(byte[] buffer) {
+        StringBuilder builder = new StringBuilder();
+        for (int i = 0; i < buffer.length; i++) {
+            if (i > 0) {
+                builder.append(':');
+            }
+            builder.append(Integer.toHexString(buffer[i] & 0xFF));
+        }
+        return builder.toString().toUpperCase();
+    }
+}