IllegalArgumentException occuring while flushing mail queue

[atlassian.core.task.AbstractErrorQueuedTaskQueue] handleException java.lang.IllegalArgumentE
xception: 'To' is a required field
java.lang.IllegalArgumentException: 'To' is a required field
        at com.atlassian.mail.Email.<init>(Email.java:51)
        at com.atlassian.confluence.mail.template.PreRenderedMailNotificationQueueItem.createMailObject(PreRenderedMailNotificationQueueItem.java:93)
        at com.atlassian.confluence.mail.template.AbstractMailNotificationQueueItem.send(AbstractMailNotificationQueueItem.java:103)
        at com.atlassian.confluence.mail.template.PreRenderedMailNotificationQueueItem.send(PreRenderedMailNotificationQueueItem.java:88)
        at com.atlassian.confluence.mail.template.AbstractMailNotificationQueueItem.execute(AbstractMailNotificationQueueItem.java:96)
        at com.atlassian.core.task.AbstractErrorQueuedTaskQueue$TaskDecorator.execute(AbstractErrorQueuedTaskQueue.java:107)
        at com.atlassian.core.task.AbstractTaskQueue.flush(AbstractTaskQueue.java:45)
        at com.atlassian.core.task.AbstractErrorQueuedTaskQueue.flush(AbstractErrorQueuedTaskQueue.java:37)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:287)
        at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:165)
        at $Proxy45.flush(Unknown Source)
        at com.atlassian.quartz.jobs.TaskQueueFlushJob.doExecute(TaskQueueFlushJob.java:27)
        at com.atlassian.quartz.jobs.AbstractJob.executeInternal(AbstractJob.java:87)
        at org.springframework.scheduling.quartz.QuartzJobBean.execute(QuartzJobBean.java:90)
        at com.atlassian.confluence.setup.quartz.DelegatingClusterAwareQuartzJobBean.executeJob(DelegatingClusterAwareQuartzJobBean.java:16)
        at com.atlassian.confluence.setup.quartz.AbstractClusterAwareQuartzJobBean.executeInternal(AbstractClusterAwareQuartzJobBean.java:44)
        at org.springframework.scheduling.quartz.QuartzJobBean.execute(QuartzJobBean.java:90)
        at org.quartz.core.JobRunShell.run(JobRunShell.java:203)
        at org.quartz.simpl.SimpleThreadPool$WorkerThread.run(SimpleThreadPool.java:520)