We’re in the process of making some back-end changes on our outbound SMTP servers. This is resulting in using some different IP addresses when we send emails.

Unfortunately, this can have some undesired effects with Microsoft as it can trigger their anti-spam systems, and being a big ugly company, they don’t have to play nicely with the rest of us, and all we can do is sit and wait for them to realise that these new IPs are delivering legitimate emails and to adjust their policy. We have no idea how long this can take.

The long and short of it is that some emails to Microsoft recipients are stuck in outbound queues and may take a few hours to be delivered. Meanwhile, we’ve balanced traffic back towards our old servers while we wait for Microsoft to alter their policies.

We will of course continue to monitor the situation.