Your server needs to be available through a permanent Internet connection with a fixed IP address. In theory, it is possible to run an e-mail server with a non-fixed (dynamic) IP address but it will not be reliable when the IP address is changed, and you will risk losing messages. With a dynamic IP address, you will also face a bigger risk of being put on one of the blacklists for dynamic IP address ranges.
If you are serious about running an e-mail server, get a decent business-class Internet connection. These are relatively inexpensive these days, and investing in one will save a lot of trouble later on. E-mail traffic does not depend on high bandwidth, so the capacity of a simple DSL line should be more than adequate.
Even though you will need a fixed IP address, you do not necessarily need a public IP address dedicated to the mail server. If your company only has a few external IP addresses and uses private RFC 1918 addresses (192.168.x.y) on the inside with a Network Address Translation ( NAT) router, this is not a problem. The NAT router connects the private network to the rest of the world, and it is possible to set up the router to forward the ports required by the e-mail services to the internal e-mail server.
The next table shows which TCP ports are most likely to be used for this.
|
Port |
Service |
|---|---|
|
|
Simple Mail Transfer Protocol (SMTP) |
|
|
Post Office Protocol (POP) |
|
|
Internet Message Access Protocol (IMAP) |
|
|
IMAP over TLS |
If employees want to access their messages from home or from the road, all that is required is to make sure that no firewall is blocking access to the required ports, and that the NAT router (if any) forwards these ports correctly. If users want to send messages via the e-mail server, some extra configuration will be necessary to allow the host to perform authentication to prevent unregistered users sending e-mail.