Problem in building Email interface using AutoComplete Extender

Question

I'm building an ASP.Net application which has an Email interface through which the user can soot mails. However, the problem is with the fields - "To:", "Cc:", "Bcc".
 I'm using Auto-complete Extender for all 3 of them. But when a delimiter( , )in this case; is added to separate multiple email address then the auto list which drops-down also contains the previous items added separated by a (,). What I want to do actually is what the email interface like yahoo / gmail / rediffmail / hotmail, etc support.
eg: When you type in the subsequent alphabets of the email address you want to insert in either of the 3 fields then they appear alphabetically. However, once you insert the one & try to insert next then no delimiter is shown in the drop-down list. They appear as just before the first address insertion without a delimiter.
            Please guide me on this issue.