Check MX settings by entering your domain name in this MX checker provided by Google:https://toolbox.googleapps.com/apps/checkmx/check
It will bring up a list of all issues detected with the configuration of your domain with links to Google help center articles.
For example, if you have not set-up an SPF record your emails can be flagged as spam.
Google states "You can set up an SPF record to prevent spammers from using your domain to send unauthorized emails, also called spoofing. Some mail recipients require SPF. If you don’t add an SPF record for your domain, your messages can be marked as spam or even bounce back."