DNS records for this domain are configured in a loop

Hello,

On-premise Exchange 2013  : User A
Office 365                                 : User B

User B is able to email User A fine.
User B can email anyone else (on-premise or office 365 or external)
User A can email anyone else (On-premise or external or Office 365, other than UserA)

Everyone else can email each other or external and receive emails fine.

User A to User B causes the error:

Remote Server returned '< #5.4.4 smtp;554 5.4.4 SMTPSEND.DNS.MxLoopback; DNS records for this domain are configured in a loop>'

Message trace on Exchange 2013 shows the message being Submitted but nothing beyond that. MessageTrackingLog shows the below error as well.

RecipientStatus         : {[{LRT=};{LED=554 5.4.4 SMTPSEND.DNS.MxLoopback; DNS records for this domain are configured in a loop};{FQDN=};{IP=}]}

All mails are delivered to on-premise, ie. MX is pointed to on-premise. I have recreated UserA's mailbox but no change. Still happens. 

Any suggestions?

Regards,
Ramu V Ramanan