PowerMail only receives email. Sending email is a well solved problem. When email needs to be forwarded, PowerMail needs help from an outgoing mailserver.
Furthermore, PowerMail will not satisfy the demanding UNIX user. There is no procmail support, for example. PowerMail is a low-cost bulk email hosting solution. If more functionality is needed within domains served by PowerMail, the use of email forwarding is suggested. For example, the domain powerdns.com is hosted on PowerMail. Yet <support@powerdns.com> is a Mailman mailinglist. This is achieved by forwarding <support@powerdns.com> to <support@mailman.powerdns.com>.
Currently, there is also no IMAP support but that will change soon.
Furthermore, it is also expected that support will be added for piping mail through programs like Amavis and SpamAssassin to mitigate viruses and spam.