SymmetricDS报错，但还能正常同步数据和发送报警邮件，请问这是什么原因？该怎么解决？谢谢。

[corp-000] - MonitorJob -
java.lang.NullPointerException
at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTransport.java:1084)
at org.jumpmind.symmetric.service.impl.MailService.sendEmail(MailService.java:108)
at org.jumpmind.symmetric.service.impl.MailService.sendEmail(MailService.java:58)
at org.jumpmind.symmetric.notification.NotificationTypeEmail.notify(NotificationTypeEmail.java:88)
at org.jumpmind.symmetric.service.impl.MonitorService.update(MonitorService.java:163)
at org.jumpmind.symmetric.job.MonitorJob.doJob(MonitorJob.java:47)
at org.jumpmind.symmetric.job.AbstractJob.invoke(AbstractJob.java:175)
at org.jumpmind.symmetric.job.AbstractJob.run(AbstractJob.java:219)
at org.springframework.scheduling.support.DelegatingErrorHandlingRunnable.run(DelegatingErrorHandlingRunnable.java:54)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:304)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:178)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:744)