For OS X 10.3 (Panther) and higher
POP accounts download all email to your computer, IMAP accounts store email on the mail server. IMAP is more convenient, but can be extremely slow if you keep a lot of mail in your inbox or are using a dialup connection.
When you first create your new account choose the "POP" or "IMAP" account type. If in doubt, choose POP.
Configure your Incoming Mail Server as seen in this example…

Note the unusual configuration of the User Name field: you must enter your full email address, except that you replace the @ symbol with a % sign. For example, if your email address is fred@somedomain.com enter fred%somedomain.com in the User Name field.
The Password is your email account password, not your Autograph Systems account password.
Although it is more efficient to send your outgoing email through your local ISP's mail server, many people prefer to use our mail servers instead. (See below for information on sending email through your local ISP and other alternative routes.)
All customers who send mail through our servers are required to use authentication. I recommend "MD5 Challenge Response" (aka: CRAM-MD5 and other similar names). You will need to enter your username/password exactly as it is entered in the "Incoming Mail" section of your mail program.
Configure your Outging Mail Server/Server Settings as seen in this example…

Note again the % sign in the User Name.
Changing the Server port from the default (25) to 587 is not required, but doing so will ensure that your ability to send mail is not blocked by incoming messages, a problem that happens during heavy usage times. Also, some ISPs block port 25 to cut down on junk mail; if your ISP does this, you must use port 587 to send mail through our mail servers.
This alternative allows you to send through your outgoing mail through your local service provider, which is the most efficient way to send mail. Your email is still labeled as coming from your own domain/address, so the recipient will see the correct email address.

The only difference between this configuration and the direct configuration decribed above is in the Outgoing Mail Server section. Your local ISP will have instructions on how this should be set up.
The simplest configuration is to have your incoming and outgoing mail go through your local provider. In this configuration, your email address is provided by your ISP.
In this configuration, you can still create email addresses in your own domain, but email sent to those addresses is forwarded from our servers to your local provider mailbox. (If you are already using the email address provided by your local provider, this option eliminates the need to modify your email program's setup at all.)
To turn on email forwarding, visit the Email Server Editor, enter your email address information, set the account forwarding option to "Forward", and specify the email address the mail should be sent to.