How to enable external email access

Are you using M$ Exchange?

If so you can configure OWA (Outlook Web Access). If you are using Exchange 5.5 then you must install OWA, if using Exchange 2000 then it is already installed and simply needs to be configured. You can find instructions for configuring OWA here (it is a .doc white paper)
http://support.microsoft.com/support/exchange/content/whitepapers/owaguide.doc

Remove any spaces from the above URL in order for it to work.

Alternately you can go to Technet home and search for OWA.

If you are using another form of Mail Server you should specify.

OWA is good and the only solution if your remote users don't have mobile computers (will have to settle for Internet cafes).

If your users have mobile PCs or laptops, configuring your Remote Access Service (RAS) maybe the most practical option. You will preserve security since it employs user security credentials. Once logon to server, it will not be that difficult to open the mail client and access emails.

A lot of reference materials are available for RAS, you may also check with you OS help file.