What is the correct format for a Peppol address?

Use the ISO6523 format, e.g. 0208:BE0123456789.

A Peppol address consists of two parts: the scheme and the actual identifier, separated by a colon. For the Belgian company number, the prefix 0208 is used, followed by the full company number. The company number always starts with a 0 or a 1. For example: 0208:0xxxxxxxxx or 0208:1xxxxxxxxx.