Setting up the proper SPF records for you domain will improve the deliverability of your emails.
Official Google Documentation
Add this TXT record to your DNS settings:
Content: v=spf1 include:_spf.google.com ~all
Use this tool to test your domain setup: https://toolbox.googleapps.com/apps/checkmx/
You should see this SPF setting:
If properly setup you will see it turn green: