There is a spam protection "greylisting" in our mail server. This feature essentially bounces all unknown message trying to connect to your server.
The premise is that SPAMMERs create scripts that bombard your server with emails. These messages are then bounced back to their sending email server. Greylisting is effective because spammers will not usually bother to attempt a second delivery, but legitimate e-mail servers will.
For those messages that are returned to valid email servers, the messages will be retried at least four times. If the first retry is beyond the Block period 5 minutes, and within the Pass Period 6 hours, the message is passed to the Spool and it goes through its normal processing.
The email will be received when the send server is sending in 1st retry. If the sender server set it as 30mins. Then the email will be received in 30 mins after he press the send button. However the retry time is controlled in the sender side. Sometimes it might take longer time for the retry.
A record is also created that says this is a valid email address and keeps it for 36 days (default). If another email from the same email address is received within the 36 day, record clock is reset for an additional 36 days and delivered directly to the Spool.