Accounts hosted on server20.000webhost.com are currently experiencing problems when sending emails using PHP’s mail() function
This is caused by the server IP being blacklisted and the 000webhost.com admins are already working on resolving this issue (getting the server IP off the blacklist). As soon as the the server IP has been removed from the blacklist sending mails will work fine again. There is no ETA for the fix, usually it takes between 24 to 48 hour to sort out an email blacklisting.
You can check this link to see if sending mails through server20.000webhost.com works for you or not. If using the testmail.php script please make sure to check your spam/junk mail folders as well. Some ISPs and email providers filter out mails from 000webhost.com and treat them as spam.