Postcard app Touchnote has suffered a data breach in which names, email and home addresses were stolen.
Users of the app, which turns photos taken on a smartphone into postcards, have been sent an email informing of the breach with the UK's National Cyber Crime Unit now helping with an investigation into the hack.
Since launching in 2008 about 4 million postcards have been sent via the app and, at present, the company behind it has confirmed it is unsure how many customers have been affected.
It has, however, confirmed "none of the data that may have been accessed is financially sensitive" as the app only stores the final four digits of credit and debit cards used.
It added, as far as it was aware, passwords had also not been taken as security techniques used make them difficult to decrypt.
Despite this, it advised customers to change their passwords and said further updates on the hack would be issued via its official Twitter feed.