You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: main.rs
    path of file: ./repos/kerkour.com/blog/2021/2021_03_16_how_to_send_emails_in_rust/src
    
    the code of the file until where you have to start completion: // use lettre::{
//     transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncTransport, Message,
//     Tokio1Executor,
// };

// #[tokio::main]
// async fn main() -> Result<(), Box<dyn std::error::Error>> {
//     let smtp_credentials =
//         Credentials::new("smtp_username".to_str
    
// async fn main() -> Result<(), Box<dyn std::error::Error>> {
//     let smtp_credentials =
//         Credentials::new("smtp_username".to_string(), "smtp_password".to_string());

//     let mailer = AsyncSmtpTransport::<Tokio1Executor>::relay("smtp.email.com")?
//         .credentials(smtp_credentials)
//         .build();

//     let from = "Hello World <hello@world.com>";
//     let to = "42 <42@42.com>";
//     let subject = "Hello World";
//     let body = "<h1>Hello World</h1>".to_string();

//     send_email_smtp(&mailer, from, to, subject, body).await
// }