Why would wp_mail think it’s succeeding, but not actually send mail?

I am using the wp_mail function within WordPress, but it only works sometimes. When I look at the error log, it says that mail is succeeding. This function is also being used for another email being sent – but that one works (at least sometimes, anyway).

if ( wp_mail( $emailTo, $subject, $body, $headers ) ) {
    error_log('mail success');
} else {
    error_log('mail fail');
}

What am I missing?