If you are using dynamic IP number (and many service providers will only give you a dynamic IP number unless you pay significantly more for your connection), then you have to recognise the limitations this imposes.
First of all, outbound service requests will work just fine. That is you can send email using sendmail, ftp from remote sites, finger users on other machines, browse the web etc.
However, your machine is NOT connected to the Internet 24 hours a day, nor does it have the same IP number every time it is connected. So it is impossible for you to receive email directed to your machine, set up a web or ftp server! As far as the Internet is concerned your machine does not exist as a unique, permanently contactable machine as it does not have a unique IP number.
If you set up a WWW (or any other server), it is totally unknown by any user on the Internet UNLESS they know that your machine is connected AND its actual (current) IP number. There are a number of ways they can get this info, ranging from you ringing them, sending them email to tell them or cunning use of ".plan" files on a shell account at your service provider (assuming that your provider allows shell access).
Now, for most users, this is not a problem - all that most people want is to send and receive email (using your account on your service provider) and make outbound connections to WWW, ftp and other servers on the Internet. If you MUST have inbound connections to your server, you should really get a static IP number. Alternatively you can explore the methods hinted at above...
Even for dynamic IP numbers, you can certainly configure sendmail on you machine to send out any email that you compose locally. Configuration of sendmail can be obscure and difficult - so this document does not attempt to tell you how to do this. However, you should probably configure sendmail so that your Internet service provider is designated as you "smart relay" host (the sendmail.cf DS option). (For more sendmail configuation info, see the sendmail documents - and look at the m4 configurations that come with sendmail. There is almost certain to be one there that will meet your needs).
Once you have sendmail configured, you will probably want to have sendmail dispatch any messages that have been sitting in the outbound mail queue as soon as the PPP connection comes up. To do this, add the command
sendmail -q
to your ip-up script.
Inbound email is a problem for dynamic IP numbers. The way to handle this is to:-
Whilst you can quite happily use the domain name servers located at your ISP, you can also set up a local caching only name server that is brought up by the ip-up script. The advantage of running a local (caching only) name server is that it will save you time (and bandwidth) if you frequently contact the same sites during a session.
DNS configuration for a caching only nameserver (that uses a "forwarders' line in the named.boot file pointing at your ISPs DNS) is relatively simple. The O'Reilly book (DNS and Bind) explains all you want to know about this. I also hear a rumour that there is a DNS (mini) HOWTO in production...
One point of Nettiquette: ask permission of your ISP before you start using a secondary, caching only name server in your ISP's domain. Properly configured, your DNS will not cause any problems to your ISP at all, but if you get things wrong, it can cause problems....