Goldbach Conjecture/Marginal

Goldbach's Marginal Conjecture
Every integer greater than $5$ can be written as the sum of three primes.

Also see

 * Equivalence of Goldbach Conjecture and Goldbach's Marginal Conjecture

He first made this conjecture in 1742 in a letter to Euler. The marginal conjecture was written in the margin of this letter (hence its name).