Although Tutanota does not usually provide coupons, it's possible to get a discount by applying for a yearly subscription on both the premium and pro versions, saving 16.7% every month. The same happens if you want to increase your inbox storage space or the number of available aliases, so you can get 10GB extra for as low as $2.65 per month or twenty aliases for $1.32 per month.
Use Tutanota with all of its security features for free, such as end-to-end encryption, encrypted calendar, and encrypted contact list.