SMTP backlog occurred in Event Diagnostic queue

Hi All,

Any one can advise for below issue .

I was facing below SMTP backlog error . SMTP server working fine, but Some of the SMTP backlog occurred in Event Diagnostic queue and stayed until system restart .
Below mentioned is app-log:
--------------------------
Debug Workflow Starting workflow Simple Workflow (776267010) in response to MailTopic: from=donotreply@gmail.com=XXXX Advisor , to=XXXXXX@gmail.com=XXXXXZZZZ , html=true, subject=Shipment YYYYYY-000353 - Agent SHIPMENT_BOOKING_TRIGGERED_XYZ begins [utility - 3]
Debug Workflow Starting workflow Simple Workflow (776263493) in response to MailTopic: from=donotreply@dgmail=XXXX Advisor , to=null, html=true, subject=Shipment XXXXXX-000353 - Agent SHIPMENT_BOOKING_TRIGGERED_XYZ begins [utility - 1]
Debug Workflow Completing workflow Simple Workflow (776267010) [utility - 3]
Debug Workflow Completing workflow Simple Workflow (776263493) [utility - 1]
Error Exception java.lang.NullPointerException

java.lang.NullPointerException
at javax.mail.internet.InternetAddress.toString(Inter netAddress.java:408)
at javax.mail.internet.InternetAddress.toString(Inter netAddress.java:373)
at javax.mail.internet.MimeMessage.setAddressHeader(M imeMessage.java:663)
at javax.mail.internet.MimeMessage.setRecipients(Mime Message.java:542)
at glog.util.mail.MailEvent$Listener.trigger(MailEven t.java:955)
at glog.util.event.MemoryEventQueueRunnable.processEv ent(MemoryEventQueueRunnable.java:145)
at glog.util.event.MemoryEventQueueRunnable.run(Memor yEventQueueRunnable.java:100)
at glog.util.event.EventThread.run(EventThread.java:8 6)
at java.lang.Thread.run(Thread.java:662)
[transport - SMTP - 1]
Debug Workflow Starting workflow Simple Workflow (778439312) in response to MailTopic: from=donotreply@gmail.com=ABC

Thanks
senthil