Can't deliver mail with response 550 Sender Unknown

I have a user trying to send to one particular address which refuses to accept the mail with the following response:

undeliverable to [recipient address]

Server response to MAIL FROM:
550 <[sender address]>, Sender unknown.

masquerade domain is same as nslookup for the ip number.

this is a piece of the smtp logs, firts the logs for the message that failes and after that logs of a message that can be delivered.
fail:
#Fields: date time c-ip cs-method cs-uri-query sc-status sc-bytes cs-bytes time-taken cs-version
2006-07-07 10:15:30 212.123.1.74 - 220+whalestone.com+ESMTP+MDaemon+8.1.4;+Fri,+07+Jul+2006+12:15:14++0200 0 71 0 47 SMTP
2006-07-07 10:15:30 212.123.1.74 EHLO mail.neeeds.com 0 4 0 47 SMTP
2006-07-07 10:15:30 212.123.1.74 - 250-whalestone.com+Hello+mail.neeeds.com,+pleased+to+meet+you 0 61 0 78 SMTP
2006-07-07 10:15:30 212.123.1.74 - 250-ETRN 0 8 0 297 SMTP
2006-07-07 10:15:30 212.123.1.74 MAIL FROM:<Admin@neeeds.com> 0 4 0 297 SMTP
2006-07-07 10:15:30 212.123.1.74 - 550+<Admin@neeeds.com>,+Sender+unknown 0 46 0 328 SMTP
2006-07-07 10:15:30 212.123.1.74 QUIT - 0 4 0 422 SMTP
2006-07-07 10:15:30 212.123.1.74 - 221+See+ya+in+cyberspace 0 24 0 438 SMTP
2006-07-07 10:16:16 80.242.238.134 QUIT - 240 524 4 781 SMTP

Notice the ETRN line.. what does it do there?

succes delivery:
2006-07-07 10:17:10 80.242.238.146 - 220+caius.dlwhosting.nl+(IMail+8.21+34366-5)+NT-ESMTP+Server+X1 0 63 0 0 SMTP
2006-07-07 10:17:10 80.242.238.146 EHLO mail.neeeds.com 0 4 0 0 SMTP
2006-07-07 10:17:10 80.242.238.146 - 250-caius.dlwhosting.nl+says+hello 0 34 0 0 SMTP
2006-07-07 10:17:10 80.242.238.146 MAIL FROM:<Admin@neeeds.com> 0 4 0 0 SMTP
2006-07-07 10:17:10 80.242.238.146 - 250+ok 0 6 0 0 SMTP
2006-07-07 10:17:10 80.242.238.146 RCPT TO:<me@soko.nl> 0 4 0 0 SMTP
2006-07-07 10:17:10 80.242.238.146 - 250+ok+its+for+<me@soko.nl> 0 32 0 0 SMTP
2006-07-07 10:17:10 80.242.238.146 DATA - 0 4 0 0 SMTP
2006-07-07 10:17:10 80.242.238.146 - 354+ok,+send+it;+end+with+<CRLF>.<CRLF> 0 39 0 16 SMTP
2006-07-07 10:17:10 80.242.238.146 - 250+Message+queued 0 18 0 16 SMTP
2006-07-07 10:17:10 80.242.238.146 QUIT - 0 4 0 16 SMTP
2006-07-07 10:17:10 80.242.238.146 - 221+Goodbye 0 11 0 16 SMTP
2006-07-07 10:18:17 80.242.238.134 QUIT - 240 639 4 782 SMTP
2006-07-07 10:20:18 80.242.238.134 QUIT - 240 754 4 781 SMTP

I wasn't and am not able to contact the admin of the receiving mail server.

Any help is appreciated to solve this asap.
LVL 1
Richard FrankSystems AdministratorAsked:
Who is Participating?
 
SembeeCommented:
I presume that you are neeeds.com ?

Everything looks fine on the DNS report.
http://www.dnsreport.com/tools/dnsreport.ch?domain=neeeds.com

And the IP address isn't listed either
http://www.robtex.com/rbls/80.242.238.152.html

This is something that the administrator of the email server needs to answer - as I cannot see anything wrong.
If this causing a business issue, then get your director to speak to someone high up at their end.

Simon.
0
 
Richard FrankSystems AdministratorAuthor Commented:
thnx Simon,

your presumption is correct. you confirm my thoughts, that the problem isn't with my mail server.

I wait for a little while and if no one hasn't anything to add to it, you'll get the points.

Regards,
Richard
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