The great thing about IMAP is that all your email is stored centrally on the FastMail servers. The biggest advantage is that you can see your Inbox, Sent, and other folders from any location. This includes the web interface, your favorite email program or mobile device. Unfortunately, some email applications don't have great support for saving the email you send through the FastMail server.
Many newer email clients (Microsoft's Mail, Apple Mail, Thunderbird, Outlook 2007) all support saving to a Sent Items folder. Unfortunately, if you are using an older version of Outlook, or perhaps a mobile device) they do not support this capability. That would mean that all your email sent from Outlook never gets saved to our server. (This is also important if you use POP, because rarely do these programs allow saving to a server)
Fortunately, we've got an easy way to enable this. It saves you email bandwidth as well (even if you are using an email client that supports saving to Sent).
How does this work? By using a feature in the FastMail Personalities option page.
One note, if you've already told the email client to save a copy to the Sent Items folder, enabling this feature would result in 2 copies appearing in the folder. Be sure to disable this setting in your email client if you intend to take advantage of this feature.
You can also define easy ways to automatically BCC a group of addresses. Personalities make this very simple to accomplish.
To learn more, I recommend this article