fixed testConnection method for mail sender if oauth2 enabled

application/src/main/java/org/thingsboard/server/service/mail/TbMailSender.java

@@ -35,6 +35,7 @@ import org.thingsboard.server.common.data.id.TenantId;
 import org.thingsboard.server.common.data.mail.MailOauth2Provider;
 import org.thingsboard.server.dao.exception.IncorrectParameterException;
 
+import javax.mail.Transport;
 import javax.mail.internet.MimeMessage;
 import java.time.Duration;
 import java.time.Instant;
@@ -80,6 +81,16 @@ public class TbMailSender extends JavaMailSenderImpl {
         super.doSend(mimeMessages, originalMessages);
     }
 
+    @SneakyThrows
+    @Override
+    public void testConnection() {
+        if (oauth2Enabled && (System.currentTimeMillis() > tokenExpires)){
+            refreshAccessToken();
+            setPassword(accessToken);
+        }
+        super.testConnection();
+    }
+
     private Properties createJavaMailProperties(JsonNode jsonConfig) {
         Properties javaMailProperties = new Properties();
         String protocol = jsonConfig.get("smtpProtocol").asText();