i have had this problem for the last three months! The quickest way round this problem i found to setup my family's email was to use the free pop servers online!
www.bluebottle.com is the best i've found its spam system makes virgins look silly! and its add free.
www.lavabit.com nice easy add free
www.inbox.com some adds in email but online file storage included.
All offer free services or you can pay to increase your mail storage!
All are pop3/smtp so you can access them from outlook ect.
setup as many boxes as you like and never get an invalid cos.domain
Hope this can be of use to someone!
