Not able to send mail to AOL email addresses

Not able to send mail to AOL email addresses

I am unable to send email to AOL email addresses. I can send and receive email from all other email addresses. There isn't an issue with AOL because I can send email to AOL email addresses using my gmail address.

Every time I send an email, i get the below error message:

----- The following addresses had fatal errors ----- 
[Status: Error, Address: <abc @aol.com >, ResponseCode 421, mtaiw-mbc01.mx.aol.com Service unavailable - try again later]