Don't have a solution, but doesn't look too good. You need more horse power for the spamassassin processes, but you don't have it. In the mean time more mails are arriving, requesting for even more cpu resources.
Maybe stop the MTA (Postfix ?) temporary until all mail is scanned to fix this immediate problem? The fact that the MTA is not available should not lead to missing emails, as normal servers wild do retry's (up till 3-5 days). More permanent investigation should probably be done.