Greylisting is a technique used to reduce viruses, spam, and other malicious traffic by attempting to determine whether or not a remote host is a legitimate mail server or simply a compromised zombie used for sending malware.  ABI Hosting's email servers do not greylist all incoming connections.  Only email coming from hosts which match certain suspicious criteria are subject to this test.  Therefore, if an email server is properly configured the likelihood of delays is very small.

More information can be found at http://www.greylisting.org/