Unable to send /recieve notification

Unable to send /recieve notification

We have problem send/recieve notification through SD+
All our settings are fine... and our exchange server running fine,
Here is the message from event log

Exception while trying to send notification for Request ID : 5 Mail sending failed.

we are running with sd+ 6
Version : 6.0.0
Build Number : 6003

Please advise us..









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