#5.7.1 smtp;550 5.7.1 Unable to relay for username@externaldomain.com error

Good morning,

The company I work at is running exchange, for the most part everything is working fine with sending and receiving mail from internal and external sources however there is one external customer which we cannot email... (Note that previously we were running postfix in Linux and were able to email this customer fine) We get an undeliverable as such:

You do not have permission to send to this recipient.  For assistance, contact your system administrator. <MAIL.OURDOMAIN.com #5.7.1 smtp;550 5.7.1 Unable to relay for username@externaldomain.com error

We have contacted their IT staff but they deny that it is their issue saying they have us white listed and the whole internal network and authenticated users have permission to relay, so I am stuck on this problem. If anyone could help it would be greatly appreciated.

Thanks For Help

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Have you tried an Telnet SMTP test to their mail server from your mail server?
Loads of guides on how to do this: http://www.google.co.uk/search?hl=en&q=telnet+SMTP+test
If that rejects you for some reason then you should get back an error message.

Are you sending email directly, or relaying through another machine with an SMTP connector?

Exchange version?
Host Windows version?

You do know there is a dedicated Exchange TA? http://www.experts-exchange.com/Networking/Email_Groupware/Exchange_Server/
It is very quiet in this general email/groupware group.



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