Symptom:  The first time you email someone the delivery is delayed but after the first message is received all other new messages to them are quick.

Cause:   Greylisting is designed to defend e-mail users against e-mail spam. It will "temporarily reject" (Delay) any email from a sender it does not recognize. If the mail is legitimate, the originating server will try again to send it later, at which time the destination will accept it and whitelist (greylist bypass) the sending email user for 90 days. So the greylist “Delay” only happens once per unique sender per 90 days. If the mail is from a spammer, it will probably not be retried thus lowering spam dramatically.

Details:  Internet specifications suggest that messages temporarily refused be redelivered within 4 hours, and most servers are configured to retry in far less time - often on the order of 5 to 15 minutes. The specific delay will depend on the configuration of the sender's e-mail servers. Internet specification requires that when a mail server receives a "400-level" error, it must queue the e-mail message and try later to deliver it. For legitimate e-mail, this process is standard and mandatory. Properly configured mail servers will redeliver their messages appropriately and greylisting should not represent a delivery challenge to them. Because SPAMmers send hundreds of thousands of e-mails per day to addresses they do not know to be working, they generate a large number of bounced messages. Acknowledging server responses for these messages, storing the messages on a server for some period of time, and redelivering them again represents for SPAMmers a resource-intensive process that might very well not return sales of their products or services. As a result, they intentionally misconfigure their mail servers. So by requiring that every incoming e-mail message comes from a properly configured mail server, most SPAM is filtered.

Prevention:  The Easiest solution to prevent any Greylisting, Spam Blocks, etc. from parties you know are legit is to add the Senders email address or domain as a Trusted Sender.  See the following article on how to do this, http://support.colony1.net/KB/a186/how-to-create-a-trusted-sender.aspx