This incident has been resolved.

We're aware of an issue affecting some customers who are attempting to send emails to external addresses. As a result of a false-positive inclusion of one of our IP addresses in a blocklist, connections to some mail services are being rejected. The IP in question has since been removed from the list, but it may take several hours for this to propagate to other mail services. We are continuing to monitor this issue and will update when we're comfortable that this has been fully resolved.