I'm stumped. I've created hundreds of user accounts and allowed people the ability to send as that user. However I just set one up and it is not working. I created an account "firstname.lastname@example.org". I added 3 users to the security tabe and gave them send as rights. I also added them to the exchange advanced, mailbox rights for read and full mailbox rights. For some reason they still cannot send as this user. System undeliverable everytime. However it works for me... and I cannot see any difference.