The answer to this question depends on whether you are a verified landlord or not. What is the difference between the "Classified" and "Verified" system?
If you are a non verified landlord you, or the interested user, must have an active premium subscription so that your contact details be visible. You both will be able to get in touch directly over mail or telephone.
If you are a verified user, the potential tenants will be able to write you for free over the Erasmusu messaging system. However no contact details can be shared for safety until the booking has been completed.