My users are trying to send mail to one particular user and every time they do they get a 550 error back Host Unknown. Mail can successfully be sent to this individual with outside email accounts (i.e. Gmail, Yahoo).

Here is an entry from my log files:

Code:
10:46:05 F46C MSG 721551 File: /opt/novell/groupwise/domaindir/wpgate/gwia/wpcsout/gwi949e/4/41541f6d.7C0 Message Id: (515465B9.EDA:232:52645) Size: 1.1 K
10:46:05 F46C MSG 721551 Sender: bkesting@cityofwayne.org
10:46:05 F46C MSG 721551 Converting message to MIME: /opt/novell/groupwise/domaindir/wpgate/gwia/send/x1541f6d.382
10:46:05 F46C MSG 721551 Recipient: paul.davis@dgr.com
10:46:05 F46C Switching link state to SENDING_ALLOWED.

10:46:05 F46C MSG 721551 Queuing message to daemon: /opt/novell/groupwise/domaindir/wpgate/gwia/send/s1541f6d.382
10:46:05 F443 DMN: MSG 721551 Sending file: /opt/novell/groupwise/domaindir/wpgate/gwia/send/p1541f6d.382
10:46:05 F443 NgwResQuery(dgr.com, 1, 15)
10:46:05 F443 Querying server (# 1) address = 198.133.246.12
10:46:05 F443 HEADER:
10:46:05 F443   opcode = QUERY, id = 62882, rcode = NOERROR, flags:  qr rd ra
10:46:05 F443   query = 1, answer = 1, authority = 2, additional = 2
10:46:05 F443 
10:46:05 F443 QUESTIONS:
10:46:05 F443 	dgr.com, type = MX, class = IN
10:46:05 F443 
10:46:05 F443 ANSWERS:
10:46:05 F443 	dgr.com
10:46:05 F443 	type = MX, class = IN, ttl = 1 hour 36 mins 53 secs, dlen = 28
10:46:05 F443 	preference = 0, name = dgr-com.mail.eo.outlook.com
10:46:05 F443 
10:46:05 F443 NAME SERVERS:
10:46:05 F443 	dgr.com
10:46:05 F443 	type = NS, class = IN, ttl = 1 day 23 hours 36 mins 53 secs, dlen = 16
10:46:05 F443 	domain name = ns40.worldnic.com
10:46:05 F443 
10:46:05 F443 	dgr.com
10:46:05 F443 	type = NS, class = IN, ttl = 1 day 23 hours 36 mins 53 secs, dlen = 7
10:46:05 F443 	domain name = ns39.worldnic.com
10:46:05 F443 
10:46:05 F443 ADDITIONAL RECORDS:
10:46:05 F443 	ns39.worldnic.com
10:46:05 F443 	type = A, class = IN, ttl = 1 day 23 hours 36 mins 53 secs, dlen = 4
10:46:05 F443 	internet address = 205.178.190.20
10:46:05 F443 
10:46:05 F443 	ns40.worldnic.com
10:46:05 F443 	type = A, class = IN, ttl = 1 day 23 hours 36 mins 53 secs, dlen = 4
10:46:05 F443 	internet address = 206.188.198.20
10:46:05 F443 
10:46:05 F443 DMN: MSG 721551 Attempting to connect to dgr-com.mail.eo.outlook.com 
10:46:13 F47C Negotiate SSL on socket: 15 Protocol Agent(HTTP)
10:46:13 F4A4 Starting GWHTTP-Handler_3
10:46:14 F47C Negotiated SSL successfully on socket:15 with 192.168.3.61 Protocol Agent(HTTP)
10:46:15 F443 DMN: MSG 721551 dgr-com.mail.eo.outlook.com connection failure (8906)
10:46:15 F443 DMN: MSG 721551 Send Failure: 550 Host unknown (dgr.com)
10:46:25 F4F6 MSG 721551 Analyzing result file: /opt/novell/groupwise/domaindir/wpgate/gwia/result/r1541f6d.382
10:46:25 F4F6 MSG 721551 Detected error on SMTP command
10:46:25 F4F6 MSG 721551  Command:  dgr.com
10:46:25 F4F6 MSG 721551  Response: 550 Host unknown (dgr.com)
10:46:25 F4F6 Building undeliverable message
What would you guys suspect the issue is?

Thanks