Ignore "invalid" or incorrectly formatted Emails addresses (on Signup/User Import)
Better will assign some email (e.g. unknown_XXX@garbage.com, where XXX is row number of csv) to email of unassigned users. There's tonn of software that work with CSV. 
After importing, you can run sql update command that will assign to emails '' value for that garbage email.

p.s. Also you can setup Rules module, that check if email equal to garbage email and warn him to change his email.