Can I access my email via my iPhone?

The iPhone supports retrieving email via IMAP, the same standard used by Outlook Express, Thunderbird, and dozens of other mail clients. So short answer, yes, the iPhone can send/receive email to/from an account hosted on one our servers. :)

To setup your email account for access via your iPhone you would go to Settings > Mail > Accounts > Add Account. When prompted for the type of mail account you are setting up choose "Other" as the mail type, then use the same inbound and outbound mail settings (IMAP, SMTP, and mail.yourdomain.com) as you do for Outlook Express or your regular mail software.

(The exact settings for mail clients can be found in cPanel by going to "Mail Accounts -> Configure Mail Client", and it will display the settings for a specific account)