This problem/fix should be here since it oddly takes a long time to find the answer on the internet, yet the problem seems common. If you get mail() not supported in this PHP buildwhen trying to use this function then you need to recompile. This happens during configure if /sbin/sendmail is not found. For some reason PHP decides to leave out mail support entirely if it can't find this file while configuring for build. If you run qmail or some other sendmail replacement it is very likely this could be your situation. To fix, make sure the appropriate file or symlink exists. To be safe, why not make a symlink from /usr/sbin/sendmail too. In the case of qmail, these should both point to /var/qmail/bin/sendmail which is qmail's sendmail wrapper. After fixing that, delete config.cache and start your build over again and reinstall. Thanks to Surojit Niyogi -- and Google, for caching the page on the www.bajaar.com where he posted this simple solution which was unfortunately broken at the time I was looking for an answer.
echo function_exists('mail')