Is it necessary to run the sendmail demon if you can use the mail features of
Mozilla or Netscape? It doesn't appear to be necessary unless you are on
a local network without other mail facilities.

I've had some trouble lately with port probing and I decided to turn off
sendmail to avoid havind the port open all the time. I seem to be able to
send and receive mail with Mozilla or Netscape despite this.

Am I turning off a facility which I might want to use later or which is
required for other operations?

I have never really understood all of the Email workings which come
with Linux.

you don't need send mail for receiving mail.
Netscape Composer, Mozilla Mail, kMail, Gmail, pine (etc!!) are mail CLIENT, so you could connect to mail SERVER with them... You probably only want to connect to your ISP smtp/pop mail server to send stuff and receive stuff... you don't need send mail for that.

Send mail is a mail SERVER so you can send mail directly from your computer, to your local network for example or directly to internet (but your ISP probably block smtp port). I highly suggest you to turn off send mail since it's a dangerous security hole... If you want to learn about mail server, try qmail