IAS 38: para 9: “Common examples of items encompassed by these broad headings are computer software, patents, copyrights, motion picture films, CUSTOMER LISTS <…>”
para 63: “Internally generated brands, mastheads, publishing titles, CUSTOMER LISTS and items similar in substance shall not be recognised as intangible assets.”
If you purchase the list it can be capitalised as it will have a fair value. If it is internally generated then you cannot allocate a value and therefore it is not capitalised.