Do not save any mail?

Customer asks this question:

I've run across a couple cases now where I need to forward mail for an
account, but do not want to keep a copy in the account's mailbox. The
account still needs to be a mailbox because they need to authenticate
for outgoing mail. But I can't find a way to keep the mailbox setting,
but not a local copy of emails. Can this be done?

Anybody know a good answer or workaround?

Thanks,
Ed Greenberg