How do MailScanner_ and Exiscan compare? What are the pros and cons?

The big advantage of Exiscan is that it can reject messages at SMTP time before you have accepted responsibility for them, which means you don't have to deal with bouncing messages and thereby becoming a collateral spammer. The big advantage of MailScanner_ is that it gives you much greater control over the load on your machines. You configure it according to the maximum processing capacity of your computer and it will not exceed that; in fact because it deals with messages in batches the cost of processing a message actually goes down slightly as the load increases, because the per-batch costs are shared by more messages. With Exiscan, you have to rely on Exim's load protection mechanisms, which basically means that you have to stop accepting messages when your machine gets too loaded. This is bad if the machine happens to be an SMTP smarthost. You therefore need more overcapacity with Exiscan than with MailScanner_.