Our team sends a monthly newsletter using Marketo and we use Litmus tracking to collect data on each newsletter campaign.
With GDPR coming into effect our team is looking into anonymizing personal identifiable information inisde the Litmus analytics reports. In the past we used the merge tag ${lead.Email} to collect individual emails, but we need to update the merge tag to a unique identifier in order to not store the email addresses inside litmus. Has anyone attempted to use ${lead.Id} or any other unique identifer from Marketo? if so can you please advise what has worked for you and how it was set up. We tested ${lead.Id} but it is not working.

Any feedback will be appreciated it. Thank you.