What does Diagnostic-Code: SMTP; 573 mean? - ruby-on-rails

Question

I am using sendmail to send mail from my Rails application. But, there seems to be a error happening while sending mails to a specific user domain. The error from /var/mail/*** is as follows:
Date: Tue, 11 Oct 2011 16:33:46 GMT
From: Mail Delivery Subsystem <MAILER-DAEMON#*****>
Message-Id: <201110111633.p9BGXkpR010310#******>
To: <****#*****>
MIME-Version: 1.0
Content-Type: multipart/report; report-type=delivery-status;
boundary="p9BGXkpR010310.1318350826/******"
Subject: Returned mail: see transcript for details
Auto-Submitted: auto-generated (failure)
This is a MIME-encapsulated message
--p9BGXkpR010310.1318350826/*******
The original message was received at Tue, 11 Oct 2011 16:33:42 GMT
from localhost [127.0.0.1]
----- The following addresses had permanent fatal errors -----
<****#{domain}.com>
(reason: 573 ********#****** failed to route the address)
<*****#{domain}.com>
(reason: 573 *******#****** failed to route the address)
<*****#{domain}.com>
(reason: 573 *******#******* failed to route the address)
----- Transcript of session follows -----
... while talking to *********.com.1.0001.arsmtp.com.:
>>> MAIL From:<*******#*******> SIZE=1273
<<< 573 ********#********* failed to route the address
554 5.0.0 Service unavailable
--p9BGXkpR010310.1318350826/*******
Content-Type: message/delivery-status
Reporting-MTA: dns; StreetSense
Received-From-MTA: DNS; localhost
Arrival-Date: Tue, 11 Oct 2011 16:33:42 GMT
Final-Recipient: RFC822; ****#*******.com
Action: failed
Status: 5.0.0
Diagnostic-Code: SMTP; 573 *******#******* failed to route the address
Last-Attempt-Date: Tue, 11 Oct 2011 16:33:46 GMT
Final-Recipient: RFC822; *******#********.com
Action: failed
Status: 5.0.0
Diagnostic-Code: SMTP; 573 *******#******** failed to route the address
Last-Attempt-Date: Tue, 11 Oct 2011 16:33:46 GMT
Final-Recipient: RFC822; ******#*********.com
Action: failed
Status: 5.0.0
Diagnostic-Code: SMTP; 573 *****#******** failed to route the address
Last-Attempt-Date: Tue, 11 Oct 2011 16:33:46 GMT
The bounced back emails are all from the same domain, and mails are being successfully sent to other domains. When I looked for SMTP 573 diagnostic codes, I got this "573 Internal server error, IP address related.". But I dont know what it means. Can anyone help me?
Update
From #guenter's answer, I got these commands and I run it and got the following outputs:
# dig streetsense.com MX
; <<>> DiG 9.3.6-P1-RedHat-9.3.6-16.P1.el5 <<>> xxxxx.com MX
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 61139
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 4, ADDITIONAL: 4
;; QUESTION SECTION:
;xxxxx.com. IN MX
;; ANSWER SECTION:
xxxxx.com. 3600 IN MX 10 xxxxx.com.1.0001.arsmtp.com.
xxxxxx.com. 3600 IN MX 20 xxxxx.com.2.0001.arsmtp.com.
;; AUTHORITY SECTION:
xxxxx.com. 155394 IN NS ns04.ntiva.net.
xxxxx.com. 155394 IN NS ns01.ntiva.net.
xxxxx.com. 155394 IN NS ns02.ntiva.net.
xxxxx.com. 155394 IN NS ns03.ntiva.net.
;; ADDITIONAL SECTION:
ns01.ntiva.net. 147 IN A xxx.xx.xx.xxx
ns02.ntiva.net. 147 IN A xxx.xx.xx.xxx
ns03.ntiva.net. 147 IN A xxx.xx.xx.xxx
ns04.ntiva.net. 147 IN A xxx.xx.xx.xxx
;; Query time: 45 msec
;; SERVER: xxx.xxx.x.x#xx(xxx.xxx.x.x)
;; WHEN: Tue Oct 11 18:08:23 2011
;; MSG SIZE rcvd: 262
ping also seem to be working:
# ping xxxxxxx.com
PING xxxxxxxx.com (xxx.xx.xx.xx) xx(xx) bytes of data.
64 bytes from linux06.ntiva.com (xxx.xx.xx.xx): icmp_seq=1 ttl=53 time=29.0 ms
64 bytes from linux06.ntiva.com (xxx.xx.xx.xx): icmp_seq=2 ttl=53 time=27.4 ms
64 bytes from linux06.ntiva.com (xxx.xx.xx.xx): icmp_seq=3 ttl=53 time=27.4 ms
64 bytes from linux06.ntiva.com (xxx.xx.xx.xx): icmp_seq=4 ttl=53 time=27.4 ms
Edit
also my hostname of my server seems to be same as the domain name causing issue
# hostname
xxxxxxx
This is same as the domain xxxxxxx.com. Can this cause the issue?

Answer 1

failed to route the address is the key here.
Seems that your sendmail doesn't know how to route that address. You need some mail-debugging here. First step could be to check the DNS for MX-records for this domain - dig example.com MX.
If there is no MX record, that is the problem. If there are MX record(s), you might try to ping that hosts.