So .. How do you all deal with the fact that company laptops need to be able to send emails when they're sitting inside the company, on the network and thus able to get only the mail server's inside name resolution, and on the outside when they can only get the mail server's outside name resolution ?

We use mainly Microsoft Outlook, but we in IT also use Thunderbird.

Is there for instance a safe way in the inside DNS to resolve the outside name to the inside IP ?
We have Windows-based DNS service inside.