Emails getting rejected

Hello there,

         One of our clients emails are being rejected during the data command phase of the SMTP transcation. It looks like after the initial RCPT TO command we are receving 250ok but during the data commad transcation message labs is rejecting our message and returning SMTP error  550 b7a91550-d106-11e7-8b96-d3e765ac3bf9 Message delivery rejected  I've verified our sending IP doesn't have a negative reputation

         Could you please let me know how we could resolve this issue? I can send additional information like copy of full bounce message through private message

Thanks,

vsri