Question : Undeliverable: You are not allowed to send b/c...sending behalf of another sender without permission.

I uploaded a pst file of another user into my outlook; but under the users profile.  Now that I have removed that profile, I am now unable to send emails.  I can recieve email.  
Here is the error reply message i would get:  Delivery has failed to these recipients or distribution lists:
username
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.
Diagnostic information for administrators:

Generating server:
/O=LSM/OU=EXCHANGE ADMINISTRATIVE GROUP (FYDIBOHF23SPDLT)/CN=RECIPIENTS/CN=username
#MSEXCH:MSExchangeIS:/DC=local/DC=domainname:LS-EX1[578:0x000004DC:0x0000001D] #EX#

I did read about related solutions online but my situation is different; i am almost certain that this problem is localized in my computer.  I tried sending email from owa and all emails were succesfully delivered.  So, the solutions that are posted online , i believe,  are for a different scenario.

So, if anyone can help me fix this problem; i would greatly appreciate it.  btw, i have admin previledge if i need to go to the exchange server.

Answer : Undeliverable: You are not allowed to send b/c...sending behalf of another sender without permission.

This is the way Microsoft decribes and it's pretty much the same as described above. The above way is how I do it.

http://support.microsoft.com/kb/291626

You have to set the permissions of the second mailbox. In this method you are logged on as the owner of the second mailbox first, thus you are allowed to grant access of others to the mailbox. Then you log in as yourself and add the second mailbox to your list of exchange mailboxes in Outlook.

Hope this helps.