One common reason for receiving this return code is that the timeout value you have specified is too short for the specified domain. For example, based on domain knowledge we expect

gmail.com

domains to take several seconds to reply. If you specified a timeout value less than our experience shows is possible, we will return AIP immediately instead of waiting for your call to time out.

Another common reason for receiving this return code is that the target email domain is a “Grey List” server. Grey listing is an attempt to discourage spam. Generally this means that a given email server will refuse connections from other email servers the first time they try to connect. After some time frame (this is server specific and may span a minute or two to twenty minutes or more) if the same email server retries the connection it will be allowed. When you receive AIP, our process will continue to attempt to resolve the address, thus satisfying a grey list email server.