Update customers without email address

[HarryBayer]

Question if I use Import List to update existing customers it seems that I cannot use the Marketo ID to update fields of the customers.

I can only use the email address to update unique customers. However certain customers are imported without an email address so how can I use the Marketo ID to update the customer fields?

[SanfordWhiteman · Accepted solution]

You need to use the API to lookup people by ID (or, in theory, by a custom field you guarantee is unique).

List uploads via the UI always lookup on Email.