Here are several things you can do:
Use a new subdomain: mail1, website.com, mail2.website.com.
Use a service like Litmus to figure out if the email is likely to be marked as spam.
Usually, emails with fewer images are less likely to be treated as spam.
Offer an "Unsubscribe" button at the bottom of your email so that you can unsubscribe those who clicked it instead of them clicking the "Spam" button in Gmail.
Lastly, you can also use https://toolbox.googleapps.com/apps/checkmx/ to check your DKIM/DMARC.