Memory problems suse 9.1

Hi I have a email box where the mem usage grows by 4% a day. The only thing that we can do is to reboot the box to clear out the memory? Being Linux this should not be! A friend told be it could be with the mirrored drive. This box runs POSTFIX with 600 users (local yuk!), apache2, rysnc (not in current use for SuSE update site). 2 gig ram, 2 80 gig SIDE hardware mirror, 2 - 200 gig for backup and install drives. When the box is first started you see 600 meg of ram being used, now its at 1.3 gig being used. What can I do to see what is taking the memory? And what can I do to flush the memory usage, We also get about 7 thousand emails per day. 70% spam of course..thanks ASSP. Thanks for the help Neal