I'm currently unable to send as a mailbox and it is just showing an error that I'm unable to send on behalf of the specified user. However, I have granted myself send as permission in Exchange. I have already recreated my profile and tried a different computer.
This account was previously a domain admin and the problem started about the time it was removed from being a domain admin but it may have fixed itself for a while (I'm not 100% sure about this though).
Has anyone got any ideas about what might fix it?