Aol mail undelivered

I have a free email account with zoho and try limit my usage, but I communicate with one person who has an aol email address. Within the last few days, my email to that aol account have not been delivered. I get the following message from "mailer daemon", telling me that that the address had fatal errors (I have, of course, sent emails to this same address from zoho for many months without incident. I have searched this forum for similar problems, but all aol problems seem to have been a year or more ago. When I want to send an email to this address now, I have to send it to myself at my email address with my ISP, and forward it from there. Inconvenient, to say the least. Is there any solution to this problem? I would hate to lose my zoho account, as I have used it as the response address from many other websites. Any help appreciated.

ResponseCode 421, 4.7.1 : (DYN:T1) https://postmaster.aol.com/error-codes#421dynt1]