If one of your messages is returned as undeliverable with an error message like the following then we have been unable to deliver your message.
Your message did not reach some or all of the intended recipients.
Sent: Mon, 16 Jul 2012 19:17:22 +0100
Subject: <original message subject>
The following recipient(s) could not be reached:
...
Our mail servers will make at least 4 attempts to deliver every message. If one of these attempts fail, delivery will not be attempted for another hour. If all attempts fail, then the above error message will be returned.
In some cases, the cause of the delivery failure will be included in the error message. For further assistance, please contact our support team including the full details of the error message.