Outlook Addin - Email checker

hi, i would like to build an addin to put control on outgoing emails. I am using event handlers to capture the sent emails. Here I am reading the domains of the recipients.

Our team sends emails to n number of clients. Each client may have more than on domain address. What is the best way to maintain the client and their respective domains information. In short how can I maintain client and domain reference data.

I really appreciate the help