Owners of iPhone and / or iPod Touch, make sure when you're setting up your mail account that you choose 'Y! Mail' as the account type, rather than 'Other' if you use Xtra e-mail.
The advantage is that your mail app will be in perfect sync with the Yahoo!Xtra webmail client.
All your folders, sent items, trash etc will magically appear on your device - even sent items that you sent long before you even owned your device will be available to you. This also means that when you send an e-mail from your iPhone / iPod Touch, the email will be in the 'sent items' on webmail as well.
iPhone owners get an added bonus - Push E-mail. Your Xtra e-mails will be delivered via 'push', which means you don't have to manually check for e-mail or set your iPhone up to check every 15 minutes. If you receive a new email, it will be 'pushed', Blackberry-style, to your device automatically.
Now, if only Outlook was this good....!