Is this account brand new in Aqua?
Or has this account been working with sending in Aqua?
If the latter, please contact your Exchange administrator, since this must have been a server side change.
If the former, I've seen this happen when a mail account has an alias (e.g. the account is really "johnsmith@corp.com", with "js@corp.com" as a shorthand alias). Sometimes the alias is not allowed to send messages, by a server policy, only the main account is.