Jul 7 13:25:12 mail amavis[31507]: (!)Net::Server: 2009/07/07-13:25:12 Couldn't fork: [Can't allocate memory]\n\n at line 293 in file /usr/share/perl5/Net/Server.pm
Looks like you're running OOM
when a new mail is incomming, it sets amavisd to work, that wants to fork some processes, but it can't, because it runs out of memory.