Graylisting is a method implemented by some email servers to protect against spam. When you send an email to a person that uses a gray listing spam filter, that mail will be checked against an approved list. If the address is not on the approved list then the message will be sent back with a "try again later" message. This happens at the SMTP layer and is transparent to the end user therefore, the person you are trying to send to will not know the message failed. The receiving system will then wait to see if you try and resend the message.
Most people will re-attempt after the first failure while most automatic systems will not. If the mail is legitimate, after a delay, try again and, if sufficient time has elapsed, the email will be accepted. While this means that all mail gets delayed at least until the sender tries again however it also can prevent a lot of spam from getting through. If you find yourself getting added back to the gray list repeatedly then you will need to contact the receiving organization and ask them to look into the issue for you. There is nothing on our end to prevent them from running these kinds of filters.
One thing that may help get you through the gray list filter is creating SPF records and domain keys. Please read out article on using SPF and domain keys to combat spam for more information. If you need further assistance or have any questions concerning gray listing, please contact our support department.