This means that, for whatever reason, the server you are using to send the mail is having trouble connecting to the incoming server on the other end. Without more of the delayed message, I cannot be any more specific than that.
When this happens, the outgoing server will continue trying to connect for a set period of time. How long it will continue to try varies with the provider.
Eventually, one of two things will happen:
1. The connection will be successful. The message will then be delivered.
2. The specified time limit will be reached without a successful connection. You should receive a Delivery Failure Notification in this case.
Hope this helps!