If it was sent as a iMessage. It will sometimes do that. Usually you have a Phone # in iMessage along with any emails that you have associated for that device. It just means it skipped the primary for (What Ever Reason) and went out via email address. Could have been a temporary issue with your carrier or sometimes a reboot of your phone will fix it. It also helps if the person on the other side, adds that email to your Normal Contact Information. That way the name displays right in the messages App.
In "Send and Receive", it's the bottom part you want only the phone number checked. To send. The top part to receive can be checked for email. That shouldn't affect anything. But uncheck emails in "Start New Conversations From".