Hi there, and welcome to the forum.
I'd be VERY surprised to hear that would happen...
In the case you describe, I think those outside would get a blank reply (if Outlook didn't elect not to send it due to lack of content). And if you didn't tick the box for internal, I wouldn't think it would send to anyone at all.
Surest way to test it though, is to... err... test it. Set it up the way you describe and send yourself an email from a gmail account, and get a buddy at work to test from the internal server. You'll have your answer for certain then.