If you non-geeks don't understand what I'm going to about, don't worry. I barely understand it myself. Let us begin by discussing Port 25 and SMTP (i.e. "Simple Mail Transfer Protocol").
Port 25 is, let's say, like a radio channel that is used to send out email. Also, to send out E-mail you need to use an SMTP server.
Spammers love port 25. They scan the Internet for servers that have port 25 open and then use that server to relay their payload of spam (which you might guess is quite large). In Groupwise (the E-mail system that our firm uses) there is no way to selectively open port 25. It's either open or closed. So, while I'm out in the urban jungle, if I want to send E-mail with my Treo phone I can't use our firm's E-mail server to do it (there are workarounds but basically that's the drill). Port 25 is closed.
At home, when I'm using my laptop, if I want to send out E-mail I must use the Cox SMTP server --because Cox blocks all port 25 traffic (or something like that). Since my laptop is not always in the same computing environment (i.e. I move from inside my firm's firewall to my home) I must constantly change the SMTP server that I use in order to send out mail (thanks spammers!).
Oh, but wait! I have OS X running, which is Unix. So I can run a program called "Sendmail" and basically have my computer act as an SMTP server. But I don't know how to configure sendmail. Well, thanks to Bernard Teo, who created the Sendmail Enabler I don't have to. I can use his program to configure my computer I'm good to go and I can send mail from anywhere at any time.
Thanks Bernard. Thanks Apple OS X. Spammers, I hope you rot in Hell.