I have installed a linux server in my office to run 16 machines. Its main use will be a internal mail server but will be also running websites.

I have installed Ubuntu 9.10 server x64 and have got apache running.

I am looking for the simplest more robust solution for smtp, pop3 and imap. I have only ever used qmail before and found it a pain to configure and its getting old so I though I should probably try something new. I have not much experience with running pop3 or imap on linux so would love a suggestion on that.

Also any backup software suggestions?

Thanks

Take a look at the several "how to" at http://www.howtoforge.com/. They have "perfect" setups for this kind of server.
For the backup question, take a look at amanda (which I use at work) and bacula. They are perfect to backup several computer/servers in a network to a high density tape.

My suggestions would be:
pop3(s)/imap(s): Dovecot
smtp(tls/auth): postfix

As far as backups go-- if you're only backing up *nix workstations rsync. If you're backing up windows and *nix then amanda or bacula would be far better choices.

Backups. Check out http://www.linuxquestions.org/bookmarks/tags/backup. For general network backup of multiple machines, I like Amanda. Rsync has it's place, and I use it. But all my department backup servers run Amanda. Once it is configured and running, it just works, and it stays out of your way when it's not running. You can run backups to tape, to disk space, to the cloud.

I build critical applications (like Amanda) from source. Last I checked the ubuntu package was more than a couple of years out of date. Building from source is pretty simple, and there are howtos on the wiki -- http://wiki.zmanda.com/index.php/Quick_start, for example.

Mail. I use a combination of Sendmail, miltergreylist, mimedefang, spamassassin, and uw-imap. It's not the easiest setup to configure and might not suit your situation. I happen to share responsibilities with a senior admin who is an expert on sendmail, and we serve hundreds of users with tens of thousands of emails a day. Blocking spam without blocking legitimate email becomes a major endeavor. You'll have to keep an eye on things. Don't be an open relay. Don't accept mail except for actual real local user accounts. Require smtp auth and use ssl or tls for all connections. Use account names that aren't simple first names, and ask your users to avoid openly publishing their emails on the web. Lots of other things involved in fighting spam, but those are some of the simplest recommendations. Greylisting knocks out a large portion of spam.

I will second a recommendation for greylisting but stick with postfix as a preferred system if you're not a sendmail expert Greylisting is massive right now in reducing spam.