Recipent is unable to reply back

[blaisey]

Hi,
I tried sending email to one of my customers. He receives my emails but is unable to reply back to me. He gets the following error.

550 5.0.350 Remote server returned an error -> 550 Sender has no A, AAAA, or MX DNS records.

It is just his emails I am not able to receive. This happened all of a sudden 4 days ago while we used to exchange emails successfully before that. Could anyone help or let me know what exactly is the issue and how can I resolve it?
Any help would be appreciated.
Thank you.

[JohnDoh]

Roundcube is an IMAP client, it has nothing to do with your DNS.

If you are confident that your DNS is setup correctly and only one of your customers is seeing the error then perhaps the issue is with their DNS resolution.