Start by logging in to your GMAIL account > click the GEAR icon at the top right > then SETTINGS.
On the SETTINGS page, go to the FILTERS AND BLOCKED ADDRESSES tab then click CREATE A NEW FILTER.
Now, let's force GMAIL to not mark emails coming from email@example.com as SPAM. Let's create a filter by setting firstname.lastname@example.org in the FROM field then proceed by clicking the CREATE FILTER link.
Setup your filter upon your discretion but make sure to tick both the NEVER SEND IT TO SPAM and ALSO APPLY FILTER TO MATCHING MESSAGES checkboxes. Click the CREATE FILTER button once you're done.