First to @EmailServerNewb Sorry for running with a similar title as another of your threads, it was too good to resist!
Gmail.com … everyone’s favorite or most hated email provider.
For most spinning up Mail-in-a-Box as a private email server Gmail has become synonymous with frustration because email is delivered to the Spam box for what seems to be no rhyme or reason.
Well, thanks to a friend at a different email service provider who clued me in to a different blacklist tool to use, I have become aware of a blacklist that Gmail.com MUST be subscribed to.
Introducing Spam Eating Monkey - a blacklist which includes all NEWLY REGISTERED domains. 
I recently helped someone on the MiaB Slack channel who had two MiaB servers set up identically. Same VPS provider, same data center, everything completely the same except for the things that could NOT be the same … the domain and the IP address.
When sending to gmail.com email addresses one MiaB sends everything to Spam, the other does not.
The MiaB which everything goes to spam is using a domain name registered this week. The other is using a domain name registered just over 30 days ago.
So while not conclusive at all, this certainly is a data point that makes some sense.
Oh and the blacklist tool that includes SEM in their results is: