This really looks like a problem for the mailscanner list. I have never actually had that problem with any of my setups.
Give the mailscanner list (http://lists.mailscanner.info/mailma...fo/mailscanner
) a try, maybe someone knows what's happening on your system.
Just out of curiousity, try increasing the ram, I read somewhere about another user who had 512Mb ram and was having the same issues. He eventually increased his ram to around 768Mb and the problem went away.