assign a unique ID to each incoming email?

I would like to use procmail to create some type of ticket number to 
incoming emails. That in itself is easy, but how do I keep track of the last
assigned number across mails, so that the numbers are nicely sequenced?

TIA
Mathias

Mathias Koerber   | Tel: +65 / 471 9820    |   
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SingNet NOC       | Fax: +65 / 475 3273    |            
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