Since you have created the 'Alert email' feature, I think this would suit the visitors/buyers not the companies/agents.
The agents may not necessarily want to be updated when new properties are listed but rather the end user (the buyer/visitor) would be interested in the alert of new properties.
It would be good to allow free members to register their requirements and get email alerts on new properties and save their favourite properties, rather than them having to be agents/companies. So there would be two log in systems, one for an agent and one for the consumer.
I dont think this is currently possible but it would be a great feature if it could be done in future.
Thanks
Maria