Not able to receive Mail from a Specific domain

Dear All,

I am using Exch 2013  with 2 MBX and 2 CAS servers. Normal mail flow is working fine , we can send and receive mails from almost all the domain Except xyz.com

We can send mail to xyz.com, but not able to receive their mails.

Mail flow is working with SMART host, I checked and found that even there mails are not hitting our Smart host, I checked that they are marked under safelist on spam filter. I checked our firewall to make sure that no ip has been blocked

But They are getting the below NDR after two days, 

Remote Server returned '<[151.xxx.xxx.xxx] #5.0.0 smtp; 5.4.7 - Delivery expired 

They are using iron port as antispam , do you thing something more i can check at my end

My point is even i cant see there mails on SMART HOST , so that i can troubleshoot.