E-mail Rejection Explanation

A fairly popular technique for blocking spam is called "greylisting", and it is gaining popularity all the time. It works like this (note that greylisting is not the same as blacklisting, you are not banned in any way):

• When a message arrives it is "deferred". That is, it is rejected with a temporary result code that means "try again". Details about the message are added to the "greylist" (the sending server, the sender's address, and the recipient's address).

• The sending mail server will try again, usually immediately. Our server recognises that this message has been attempted before because it matches an entry in the greylist, and allows the message through.

• When a message is allowed through, the sending server is automatically "whitelisted", which means that subsequent messages from the same mail server will not be deferred, regardless of who the sender or recipient is.

If legitimate mail is failing and returning a message about being greylisted, then the sending mail server is broken because it is either treating a temporary result code as an error, or it is configured to not retry. Note that there are many other reasons why a temporary error might occur (like a DNS timeout, a busy server, too many connections from the same host, a full mailbox, etc.) so thse servers will break more than just greylisting.

If you are one of our customers and you are having trouble receiving mail from an ill-configured site like this, contact us and we will temporarily exclude the sender's mail server from this test.

If you are an end user of the misconfigured system then you should ask your ISP or IT department to fix it. Please do not complain to us.

If you are the administrator of a server that doesn't properly handle temporary result codes, then you should fix it.

Note to server admins: some folks argue that greylisting is "bad" because it clogs up the sending server's queues with delayed mail. While this can be true when greylisting is done poorly, it is not the case with us. For starters, we do not impose a significant time delay like most greylisting methods use. A retry even a few seconds later will succeed. Also, once a message is accepted we whitelist the entire /24 range of IP addresses. In other words, at most one or two messages are deferred for a few seconds, after that all mail flows through without delay. There simply isn't any extra work imposed on your servers.

You can read more about our e-mail practices and policies here.