[CentOS] memory leaking?? kernel problem?

List, I have the two servers CentOS 4.0 (running postfix, mailscanner) on this hardware: HP netserver 2000, PIII - 1GHz, 512 ram, two Hdd SCSI - 36 GB (RAID 1), I have monitored this server for a couple of months and I've seen that the memory used is increasing every week until it reaches all available memory and the server crashes.... at this point mailscanner crashes or sometimes the server gets freezed....my question is?

Is this a memory leak?

I 've always used this combination (mailscanner, postfix on Whitebox) and never got a problem, but when I upgrade my system to CentOS (kernel 2.6) this problem began..

can you help me?

regards

Israel