Email not working

Today I had a user forget his password and try to reset it. This error pops up:

Error : Unable to send mail.
Possible reason: your host may have disabled the mail() function.
Please contact your Piwik administrator.

I know for a fact that the PHP mail function is enabled on my server, as does Piwik according to the settings menu. Email is working fine on my other PHP apps.

I tried to test sending an email report but it just sits on “Loading data…” and never does anything.