flow session

Hi  I am using Juniper SRX240 and for last week I find records like this in session table.

Session ID: 668, Policy name: allow-public-mail/11, State: Active, Timeout: -1, Valid
In: 47.74.61.85/60414 --> x.x.x.x/587;tcp, If: reth1.0, Pkts: 1, Bytes: 40
Out: 192.168.200.16/587 --> 47.74.61.85/60414;tcp, If: reth0.994, Pkts: 6, Bytes: 264

This records stay in table forever and I have to make clear session table. Can you please explain me why the timeout is -1 ?

Thanks