BESTMED - Unprofessional and *********

As a healthcare provider, I am not contracted to Bestmed so their members pay cash to me and I claim back. Their member claimed from them and they erroneously paid the practice. They had an outdated email address for the practice and after sending an email to the incorrect address they handed us over. Had they made a concerted effort to contact me and inform me of the money owing, we would have gladly paid them back. The first I heard of it is an SMS from the collections company. Now I have said I will gladly pay but they must retract account from collections to which their response is that the onus was on me to update email address. WHY? I don't do business with them so am I supposed to update my contact details with every person in the world just in case they need to contact me. RIDICULOUS!!!