if you are running pop3 and smtp and wish to allow others to have an email address using your service out side of your local network you may run into a problem; relaying should be disallowed for others out side of your private network, so how do you allow someone with an account that you gave them to use your mail service, without opening your server to be an open relay for anyone who wants to abuse it? this website: http://popbsmtp.sourceforge.net/

provides a great perl utility that will look at successful pop3 logins and then create a temporary database with that users ipaddress and user name. with that you can set your mail server, (postfix, sendmail...) to look at the database and will allow mail relaying for those that have successful pop3 logins. this utility works with various pop3 servers, and the website includes detailed instructions on just exactly how to set it up. all youll hav eto do i tell your users to CHECK their mail first before they try to send it.