I like shift ciphers as much as the next guy but Caesar is just so damn obvious. If you’re sending a giant wall of nonsense letters to a friend in an email, you might as well wave a giant neon-colored flag around that says “HEY I’M A SHADY BASTARD” to any third parties that might stumble upon it.
A proper cipher should have some form of camouflage. Like a sweet little house on a hill. With fireflies hanging out.