The Royal Mail says it’s suffered from a disruption to its ability to ship packages or letters internationally, which the BBC says is because of a “cyber incident.” The courier is advising customers to “hold any export items” until it solves the issue.

At the moment, it’s unclear whether the incident is an actual attack from hackers or some other technical error, though the UK’s National Cyber Security Centre says it’s working with the Royal Mail “alongside the National Crime Agency, to fully understand the impact.” The BBC also says that regulators have been told about the issue.

The downtime could be a pretty big disruption — according to the BBC, the courier sends around 200,000 international parcels a day. Depending on how long it takes for the Royal Mail to restore its international export services, that could be a lot of packages and letters piling up in the backlog. Also, anytime a major piece of infrastructure goes down, there can be widespread ramifications, as we’ve just recently seen in the US with an FAA software issue affecting thousands of flights.