Twitter isn't a stranger to technical problems (many still have memories of the fail whale), but they've more recently revolved around strange behavior rather than full-fledged outages. That makes the DM failure all the more glaring -- as much as Twitter's reliability has improved over the years, it's still not immune to issues like this.

We're currently having some issues with DM delivery and notifications. We're working on a fix and will follow up as soon as we have an update for you. Apologies for the inconvenience. — Twitter Support (@TwitterSupport) July 3, 2019