My WordPress installations (more than 1 in the same webserver) have stopped sending notifcation emails (note it used work).
I tried the following PHP on the server:
$message = "Line 1\nLine 2\nLine 3";
mail('firstname.lastname@example.org', 'My Subject', $message);
and it was successful. Anybody have an idea what is going on here? Any tips for debugging this?