An point worth considering here is that sp_send_dbmail is a system stored procedure provided by MS with the product. Customizing it may result in your functionality "just going away" if you upgrade to a newer version or patch it.

Another possibility is to wrap the sp_send_dbmail proceudre and then call your procedure instead. Then have your wrapper do whatever it is that you need it to do. Example:

CREATE PROCDURE my_send_dbmail(parameters_supported_by_db_mail)

AS

BEGIN

EXECUTE sp_send_dbmail passed_parameters;

--do your stuff here

END;

Granted, you'd ideally support all the parameters that sp_send_dbmail supports and should pass them to sp_send_dbmail by name, but you'd lower your risk if you upgrade. Just something to consider.