Mail Client fails to open ("not responding")

After performing the most recent os update, my computer began failing to open the Mail app. "Mail not responding" is the indication in the activitiy monitor; no windows open after minutes; I force quit. I would like to switch my default email reader to a replacement application, Airmail, but I gather in order to do so I must be able to open Mail. Any advice on how to accomplish this would be greatly appreciated.