We are in the process of creating a user portal so our customers will be able to log into our site. We are wondering if it's possible to have our website call a Marketo API to check if the user's email address already exists in Marketo? Does anyone know if that is feasible?
Please move the question to Products
As confusing as this is, the "Community" space is for website feedback, not support topics.
After you move it, I'll answer over there.
The superficial, rather unreasoned answer is "Yes." There is a REST API endpoint to lookup lead(s) by email.
The realistic answer is that you'll be introducing a massive Denial of Service vulnerability if you expose individual API calls in this way. The only way to roll out something like this professionally is to enforce rate limits using a robust algorithm, plus daily caps to leave space for other integrations -- but even with those controls you may not have enough to service legit end users.
A more appropriate way to reapproach this goal is mirroring your Mkto database, via periodic batches, to your own database. Then offer that second db for real-time lookups. The Marketo API is good for batch/bulk operations, not one-at-a-time use.