This is because AOL requires each sending IP address to have a reverse DNS record. When sending an email from your script and using "localhost" as your mailserver host name, email will be sent from the web server's default SMTP server which doesn't have a rDNS record.
You should instead send email through our regular email server by specifying mail.yourdomain.com as your mailserver hostname. You will need to perform SMTP authentication to send email via the regular mailserver and hence you must enter the username and password for the email address from which the emails would be sent.
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