Could be a number of things, but here’s one that I’ve run into before. If it’s a reply to an e-mail received, the original e-mail may have been sent to a mailing list or separate e-mail address. Then when you hit Reply, Outlook tries to send it as if it’s “From:” the original addressee. If you don’t have “Send As” permission for that account, you can’t reply as if it’s coming from that other account. Check the “From:” field on the reply and see if Outlook has plugged a different e-mail address in there. If it has, delete the address from the “From:” field and then try to send, because then it’s being sent from the proper account that has permission to send.