This issue has now been resolved.

TOPdesk are currently experiencing problems in their UK1 hosting location: "as a result, outgoing mails may be impacted and some users could also experience performance issues. We are aware of the problem and are working on a solution."

TOPdesk have confirmed that this is a wider issue, and they are investigating.

We’re investigating an issue with TOPdesk that is impacting some users. We’re working to fix the problem as quickly as we can. We’ll share another update shortly.