To set up a mail account on your iPhone, follow the steps below (the instructions are for iPhones with software version 2.x):

• Press the Settings icon on your Dashboard
• Select Mail, Contacts, Calendars
• Select Add Account...
• For account type choose Other
• Enter the mail account information:
  • Name: the name of your mail user (e.g. John Smith)
  • Address: your mail address (e.g. user@yourdomainname.com)
  • Password: the password for your mailbox
  • Description: a short description of your mail account (e.g. John's Mail)
• Press Save
• Select the type of incoming mail server - POP or IMAP
• Enter the Incoming Mail Server details:
  • Host Name: the mail server address (e.g. mail.yourdomainname.com)
  • User Name: the mail account (e.g. user@yourdomainname.com)
  • Password: the password for the above mail address
• Enter the Outgoing Mail Server details:
  • Host Name: the mail server address (e.g. mail.yourdomainname.com)
  • User Name: the mail account (e.g. user@yourdomainname.com)
  • Password: the password for the above mail address
• Press Save
• Choose Yes when you get a message saying "Cannot Connect Using SSL. Do you want to try setting up the account without SSL?"
• The Advanced mail settings will be displayed. You can leave these settings to their default values

That's it! From now on you will be able to manage your mails directly from your iPhone.