Feature Request: SMTP

### Impacted plugin

Jetpack

### What

Add a simple SMTP feature to Jetpack. Thus, there is no need for external plugins for Jetpack users.

### How

With the addition of the SMTP feature, we, your users, will be of great convenience. It is compatible with Jetpack. Some plugins do this in a very, very small size. Jetpack should do it with this simplicity too. Just enter the necessary parts, test, ok.

For me and many others, this is necessary. If not the default, at least a plugin like Jetpack should be able to do this.

The request identifies Jetpack but does not name files, tests, or an existing entry point. First map how Jetpack currently handles mail and user-configurable settings, then define the SMTP scope, required fields, connection test behavior, and acceptance checks before implementation.

Written by the indexing model from the issue text.