If a property is constant, so created with let, you have to initialize it in place or in the init method, even if it is an Optional. If you want to be able to set email optionally, you should change let to var. In other words, if you are not initializing a variable in either the init method or class body, then the variable must be both a var and an Optional.
Related statements in the docs:
You can assign a value to a constant property at any point during initialization, as long as it is set to a definite value by the time initialization finishes. Once a constant property is assigned a value, it can’t be further modified.
For class instances, a constant property can only be modified during initialization by the class that introduces it. It cannot be modified by a subclass.