frozen emails in queues under smtp local

Our exchange server 2000  was working fine till to wensday afternoon.Then
it
started working slow.Then I got lots of emails (incoming and outgoing)
frozen
in queues. I checked the server with Virus Killer and spyware I couldn't 
locate anyone.

Then I tried to start and stop smtp service. so some of emails got
delivered
(a few)and some didn't ( lots of them ). I even deleted all emails from 
queues the restared Server. But no result, again I got lots of new emails
frozen (marked frozen). I tried unfreeze command but it didn't helped.

any idea or help would be appreciated.

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