In praise of- Nullmailer
If you run web servers that need to be able to send e-mails but don't want the hassle of running a SMTP server or are worried it may be blocked as "untrusted" if you're hosting on a ADSL line then you may be interested in nullmailer.
Postfix and exim4 do allow use of smart hosts but it's not terribly easy to set up AUTHENTICATED smart hosts and it seems to be overkill to run a full SMTP server simply to allow PHP to send e-mail.
Step in nullmailer, simple configuration for authenticated and unauthenticated smart hosts and everything is relayed via the SMTP server of your ISP or hosting company.
I'd suggest setting up a dedicated mail account for the purpose so you don't break things by changing your mail password.
Index of nullmailer
+1, it works a treat on light virtual machines. Don't install from source if you can help it though; there are packages for FreeBSD/NetBSD, Gentoo, openSUSE, Debian and Ubuntu.