This mail server uses a technique called greylisting to cut down on spam. Greylisting works by initially rejecting mail from people you haven’t received mail from before. Legitimate mail servers will attempt redelivery shortly afterward, but the vast majority of spam gets tricked by this. If you are waiting for an email from someone new, such as if you are registering on a new website and are waiting for an email confirmation, please be aware there will be a minimum of 3 minutes delay, depending on how soon the remote server attempts redelivery.
Systems Administrator