Mailx gives 550 5.7.1 Unable to relay for external email ids

[goyalpdm]

Hi-

I m having a problem with sending e-mails using mailx command. On executing the command for users in external domain, I get:

The following recipient(s) could not be reached:
'<user>@<external_domain>.com on 11/08/2007 11:11 AM
550 5.7.1 Unable to relay for <user>@<external_domain>.com


The e-mail address being used are valid and can send the email using outlook. The program works fine if send email to users within my company.

Version Details:
SunOS 5.8 Generic_117350-26 sun4u sparc SUNW,Sun-Fire-15000
mailx version 5.0 Tue Oct 9 14:37:47 PDT 2001


Please advice.
Cheers/ Sachin