A new technique called greylisting has become increasingly popular in the last few years; it involves temporarily rejecting all messages destined to a "triplet" that has never been seen. With the triplet, you identify the IP address of the connecting client, the envelope sender address, and the envelope recipient address. Once the message has been temporarily rejected, the connecting server will try to resend it according to queuing policies; when that is done the MTA accepts the message and caches the triplets for a specified amount of time (usually no more than a week). The principle is that temporarily rejecting all messages would not lose legitimate messages (since they are going to be resent by compliant MTAs) but blocks some malicious traffic since spammers don't bother resending when they get a temporary error in most cases.
Although completely transparent and theoretically not harmful, greylisting is a bad practice overall: First of all it slows down all SMTP traffic, including legitimate traffic, with a delay that could be as short as a few minutes but as long as several hours. Additionally greylisting is a queue nightmare for every mail gateway. Mail list servers and busy gateways will especially suffer in terms of resources needed to keep all the greylisted messages.
Although the caching rejects only the first message for each triplet, the amount of SMTP traffic that some servers get and the fact that, in some cases, the triplet changes very easily makes it less effective. In the worst case scenario, the retry might come from a different triplet due to IP address changes or different envelope senders (some servers employ dynamic envelope senders, especially mailing list servers), and then the legitimate message would be re-sent forever until it expired in the queue.
If you do implement greylisting, watch out for the problems it might cause and be aware that it's far from being friendly to other server queues.
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