Summary of required functionality

MUST provide a user interface for the creation of iCalendar objects

MUST parse iCalendar objects into a user-readable format

MUST be able to store a single user's calendar information in a local database

SHOULD be able to sync with servers

SHOULD be able to send email directly

but MUST be able to either sync or send email

MUST be able to store all its users' calendar information in a database

MUST explicitly know who its users are (no guessing based on domain name, because that's subject to weird human error and DoS attacks)

MUST be able to sync with clients

SHOULD be able to send email to external attendees

MAY be able to talk directly to other iCalendar servers