Early this morning we discovered a customers out-of-date WordPress installation on one of our servers had been compromised. The compromised website was then used to send out large amount of junk email.
As soon as the problem was discovered, the website was disabled and all remaining junk email removed from any outbound queues.
Unfortunately, as a result, a few external DNS Blacklists (RBLs) have temporarily listed the affected server. We have requested removal from these RBLs and are waiting for the blocks to be removed. In the short-term, some customers may find some emails to certain providers are temporarily rejected until the blacklists are fully removed.