Company I used to work for were only just completing the move from notes/domino to outlook/exchange when I left last year, there was a lot of resistance by the end users as well to the change. I was never a massive fan but it did have some useful features.
You could setup a rules to reply with a template email and create a template for each external address you want to redirect people to. The only problem with that is reply with template seems to be client side only which means outlook must be running to process the rule, not ideal if you don't...
Are you using Outlook rather than Outlook Express? Microsoft retired Express years ago, if you are using Outlook you want out of office, you can set different messages to be sent to internal and external addresses.