lettre/lettre ? reverse-engineered prompt

Build me a Rust email sending library that makes it easy to create and send messages without a lot of setup.

I want a simple builder style API for composing emails with sender, recipients, subject, headers, and plain text or Unicode content. It should support sending through different transports, especially SMTP with encryption and login, and it should also work asynchronously. Include a nice example showing how someone would send a basic email through a Gmail style SMTP server with credentials.

Make sure it feels like a real reusable crate, with good docs, tests, and a few examples. If there are tricky SMTP settings, add an auto configure example that helps figure them out from a host name. Keep it focused on sending mail, not parsing email. Please use the current Rust docs and best practices if you need to look anything up.