Hi,
I have done something similar from what that you want. This company has around 1000 invoices a month.
We had a setting on customers card and if the email was indicated it was sent to that address. It was a batch job to send or print all the posted invoices at once. The printing + sending method can also be autimatic by a function. I have done it on NAV 2009 Classic, which has a build in email sending method, all you need to have is an smtp server address and priviledges to send mails.
If you have further questions feel free to contact me.
Regards,
Marcell Major