If you have a verified email address on Celly, then when you start a new cell or when you adjust cell settings you will be presented with the option to set email domain security.
First, look under 'Membership' options. Next, select 'Email Domain'. Finally, choose 'Save'. You now have enabled email domain security.
*note that free webmail domains (i.e. gmail.com, hotmail.com, etc.) do not support email domains at this time.