NDR's - they really are most uninformative. (They should say why they were sent - like over-quota or whatever.)
Also, when an internal user sends email to an outside user and it is rejected, something other than "You do not have permission to send to this recipient" would be good - because rarely is it as simple as no permission.
Take this (munged) example:
person@example.net.nz <mailto:person@example.net.nz> on 1/04/2005 11:16 You do not have permission to send to this recipient. For assistance, contact your system administrator.
< computer.example.net.nz #5.7.1 SMTP; 550 5.7.1 Unable to relay for person@example.net.nz>
If it can't be made more specific, then make it at least less misleading
- very generic like "Unable to deliver email." I would suggest for externally destined email, something like "Unable to deliver email.
[SERVERNAME] responded with <[SMTP ERROR]>. Contact your system administrator for assistance if necessary."
Anyway, just a piece of feedback.
Thanks,
Shannon