If the agencies can use "@gmail.com" addresses it probably may help you without any additional cost. Gmail address for the agency may be for example:
Google (maybe also others too?) allows to modify externally the address (while it stay in fact the same) in following way:
"agency+1@gmail.com" - and the correspondence will come on the original "agency@gmail.com"
for next candidate can write:
"agency+2@gmail.com" and so forth with the same effect. We are using it for testing purposes to have only one address for many "ghost" candidates.
There is no need to create the agency+1 and next of course.
Probably it will help you. Regards