One of our most effective anti-spam defenses is rejecting connections (or blocking). An attempt to connect to our inbound mail servers might be rejected for a variety of reasons; the most common is that the sending IP is actively blocked from sending mail here, but there are other reasons for us to reject a connection, and we'll talk about each of them here.
Please note that while we return an error message as part of the rejection, we reject connections very early in the transaction (i.e., at connection time) and some servers miss the error message and respond to it in ways we don't intend. If you're finding that your server is having an issue connecting to ours, you may want to try some manual troubleshooting to isolate the reason for the problem. The following pages may also assist you in tracking and troubleshooting the issue: