I have an SBS 2003 / SP2 server.
There are 5 users.
They are all able to send email internally and externally without any problems.
ADOU user settings for all users email delivery restrictions - message restrictions is set to allow from everyone.
I get the following NDR when sending an appointment invitation using Outlook 2003:
You do not have permission to send to this recipient. For assistance, contact your system administrator. <mail.wellspringbaltimore.com #5.7.1>