Queue Transport - smtp

Hi!!

I have a problem:

The memory event queue called Transport - smtp only accumulate messages.
When it has about 60thousands messages OTM has out of memory errors and We have to restart it.
We really dont know why OTM is accumulating messages in that queue (Transport - smtp).
What can cause this problem ?
We already try to increase the queue threads(double) but this dont solve the problem.
We use this servlet to see queues and messages:
glog.webserver.event.EventDiagServlet



Thank you very much!!