Greetings

I have a bit of a strange one that I can't figure out. Client has a pretty standard GW2014 R2 installation on a Windows 2012 server. One domain one postoffice and GWIA. They have one particular client they email with, without issue. They also use a common accounting package that they use GWIA to relay messages off of. Setup smtp address, account and password. All works well except for this one domain. When they send invoices to this client, they get this result.

17:05:24 1520 MSG 44236 Processing inbound message: C:\GroupWise\Domain\wpgate\gwia\receive\e4408495.0 02
17:05:24 1520 MSG 44236 Sender: billing@xxxxxxxx.ca
17:05:24 1520 MSG 44236 Recipient: esg@xxxxxxxxxxxx.com
17:05:24 1520 Recipient: esg@xxxxxxxxxxxx.com
17:05:24 1330 DMN: MSG 44236 Send Failure: 550 Host unknown (xxxxxxxxxxxx.com)
17:05:34 09E8 MSG 44236 Analyzing result file: C:\GroupWise\Domain\wpgate\gwia\result\r9480454.00 3
17:05:34 09E8 MSG 44236 Detected error on SMTP command
17:05:34 09E8 MSG 44236 Command: xxxxxxxxxxxx.com
17:05:34 09E8 MSG 44236 Response: 550 Host unknown (xxxxxxxxxxxx.com)
17:05:34 09E8 Building undeliverable message
17:05:34 0D20 MTP: File sent: 4948045e.i21 Size: 2652

For the life of me, I can't figure out why sending an email to the client through the groupwise client works, but relaying off of GWIA doesn't. I've checked the usual things. NSlookup set to MX works fine. Checked DNS records on server. Seems weird that that GWIA would resolve differently depending on how the email is delivered.

Anyone have any ideas?