Why can't I use PHP's mail() function on this Bitnami server?

I've installed sendmail:

sudo apt-get install sendmail

And I've restarted the server:

sudo /opt/bitnami/ctlscript.sh restart apache

This has enabled mail() for me in the past, on non-bitnami servers.

But PHP's mail() function isn't working. We are using WordPress and need the ability to reset the password, which requires an email confirmation. Our server is an EC2 instance at Amazon.

Help!