All mail routed to this server is subject to various forms of filtering to assist in limiting the deluge of unsolicited mail from virus infected machines. It is our hope that Greylisting, which is one of the forms of filtering we use, helps to slow down the deluge.
What happens, is that each time a given mailbox receives an email from an unknown contact (ip), that mail is rejected with a "try again later"-message (This happens at the SMTP layer and is transparent to any user). This, in the short run, means that all mail gets delayed at least until the sender tries again - this is where spam loses out! Most spam is not sent out using RFC compliant MTAs; the spamming software will not try again.