Concentrate on INBOUND blocking.   If it does not get delivered to your mailbox, it does not get forwarded.   If you have more than one mailbox, inbound blocking helps all mailboxes.   

Outbound blocking should be configured to prevent an infected machine from spamming.   If you manage to avoid infected machines, you don't have to worry about outbound blocking.

But you have also identified the reason why auto-forwarding is, in my mind at least, a terrible idea.   It hinders the downstream server from making correct filtering decisions because both wanted and unwanted sources are hidden behind your forwarding server.  We can assume that the filtering rules in a downstream server will be different from your filtering rules.  If your server is too strict, you lose messages.   If your server is too lax, you get blacklisted as a spam source (and also lose messages.)   So if you insist on auto-forwarding, you should be certain that your filtering is really good before you enable that feature.