Is there a way (using postfix) to relay to specific recipients in mydestination domain?

Example:

We have our mail hosted by provider (where all the mail user@ourdomain.com is delivered).
We pop this mail with fetchmail to bring it in house (to our linux server).

We use postfix local delivery for our local mail delivery in house (mail users have unix accounts, they use pop3 to get their mail from our linux server to their mail clients on their PC's). Relaying takes care of sending email out to other domains (other than mydestination domain).

There are a couple of outside_users@ourdomain.com that don't have accounts in house and can only get their mail by pop3 directly from the provider's server. We need to have messages sent to outside_users@ourdomain.com (from our local network) relayed so that they get to the provider's server.

Any ideas?

P.S. It would also be helpful, if one could relay mail for inside_users@ourdomain.com sometimes (i.e. when an inside_user is at holiday and wants therefore his mail from our local network to be delivered to the provider's server instead of locally - as there is "no way" to access the mail stored at our linux server from the internet).