Did something change to the wp_mail() function in the last few WordPress updates that I somehow missed out on?

For some reason my simple contact form do not work anymore. No plugin, just hardcoded into a page template by the use of wp_mail().

$sent = wp_mail( $to, $subject, strip_tags( $output ), $headers );

if ( $sent ) prefix_contact_form_generate_response( "success", $message_sent );

else prefix_contact_form_generate_response( "error", $message_unsent );

I get the ELSE statement above.

Strange part is. When I test this form locally on my computer by the use of an SMTP plugin it just works and I can send emails via the form, from my localhost > via SMTP mailprovider to my inbox. Testing works fine. Online on the server is doesn’t.