Using a keyword that contains a space in a Spotlight search?
This is not an answer. I wrote this just inform:
In my experience, spotlight can get the right result, even if it contains spaces in the file name. Please refer to the screenshot below



First, Make Sure enable "Mail &amp; Message" option in spotlight preference. 

PS:Usually you can search email in build-in Apple mail search or using Smart mailbox. Of course, you can also use the spotlight to search for email.