Validate TextBox with multiple emails

We have a textbox in a FormView that we allow the user to enter multiple email addresses into and separate them with a comma or semicolon (we handle both when emailing).  I would like to validate each one entered but the validation control cannot handle more than 1 or it throws a validation error.  Is there any way other than a custom validator to do this?  If I use a custom validator then is there an expression that will validate as I pick them out of the array?  Thanks.