There can be different sources for recipient lists - it is possible to build your own to store/retrieve recipients from any data store. The internal source stores recipients in the EPiMail database and can only export and import complete lists, not edit them in place.

The only type of segmentation that you can do then within a list is to use categories. It is not the list but rather the mail sent that is categorized. Categories comes in to play when the user unsubscribes - if you send a mail categorized with X, the user can select to unsubscribe from further mails categorized with X (but not other categories) or to unsubscribe completely. Say the user chooses to unsubscribe from X, and you send a new mail with category Y, the user still gets that mail. Probably not what you are looking for, but I thought I'd try to explain it at least.

Thanks for the info Magnus, much appreciated. It all helps me build a picture of what capabilities the EPiMail feature provides.