Did you install SMTP service on your MOSS server? If not, please install SMTP service on your MOSS server.
Are the setting mode is automatic or advanced? If it is advanced, change to automatic.
Did you enable Directory Management Service? If so, check whether the application pool account for the SharePoint central administration web site has create, delete and manage user accounts right to the container in the Active Directory directory service.
Check whether the application account for central administration, the logon account for the Windows SharePoint Services Timer service, and the application pool accounts for your web application have correct permissions to the e-mail drop folder.
Did you configure MX record for your MOSS server in DNS? If not, configure correct MX record for your MOSS server and check the effect.
If you use Exchange server, you can refer to the following articles:
How to configure Email Enabled Lists in Moss2007 RTM using Exchange 2007
How to configure Email Enabled Lists in Moss2007 RTM using Exchange 2003
www.combined-knowledge.com/Downloads_2007.html