You will not be able to send it at all.
It would be clearer if sending DMs is disabled for newbies as you say, but that's at least not what it seems to be happening from the perspective of a newbie account.
I was able to reply to a DM as a newbie. The forum didn't say anything, and the message went through. It's there on my outbox.
When looking at my sent DM it seems that the forum prepends this text to it:
!!! WARNING: This user is a newbie. If you are expecting a message from a more veteran member, then this is an imposter !!!So, that's why I created this post. I'm not sure what will happen to that DM when I get to the next level.
Will it eventually become visible to the receiver?, or do I need to re-send the DM once I'm on the next level?