From: Kev on
I've been racking my brain on this subject and have read many threads
regarding the subject, but can't find a solution.

I've added another Accepting Domain to the SBS 2008 Exchange 2007.

I can receive emails from the Second Domain and it routes it to the a user
mail box.

Example

Domain1.com and Domain2.com = user1(a)domain1.com and user1(a)domain2.com

If an email is send to user1(a)domain1.com and domain2.com, user1 will receive
both emails.

If the user tries to send email from domain1.com, it sends fine
If the user tries to send email from domain2.com it produces a error

"You are not allowed to send this message because you are trying to send on
behalf of another sender without permission to do so. Please verify that you
are sending on behalf of the correct sender, or ask your system administrator
to help you get the required permission. "

When I create a new user with the email address of he Second domain, I can
send emails fine from OWA.

I apply the "SEND AS" permissions, and I can send email of user2 using the
domain1.com fine, but if I change the users email to domain2.com I receive
the same error message as above.

I have applied both Send On Behalf, and SEND AS, received the same error. I
also read that this is not a good practice, so turned off one or the other
produces the same error.

Any advice would be greatly appreciated!

P.S.
I've tried setting up a Security Group - Didn't work same problem
I've tired setting up a Second Sender Connector - Didn't work
I've tried setting up a different SMTP under the default Send Connector -
Didn't work, actual broke the sending function of SBS2008...