Ok..here's a strange one!
Exchange 2010, clients are Outlook 2007/Outlook 2101. Permissions are set the same in the EMC as delegates to "send on behalf". Both have "Full Access" permissions on this mailbox.
Both users can send an email selecting the mailbox address that they have permissions to. One shows "on behalf of"(Outlook 2007), the other shows as being sent by that mailbox (Outlook 2010). I even gave myself the same permissions as a delegate and full access permissions and using Outlook 2007, and get the same result of the email showing as being sent by that mailbox and not as "on behalf of".
I've been Googling the heck out of this, but can't find anything that seems to relate. I've even logged into a machine as that mailbox account and tried adding myself as a delegate there. I did confirm on both exchange and the client that if a delegate is added on either side it shows up correctly.
Any ideas?
Cheers,
Cameron