Is your feature request related to a problem? Please describe.
I'm finding the type bounds required to have a testable function (that accepts both async SMTP and async stub transports) somewhat clunky. I'm quite new to rust, so perhaps I'm doing it wrong, but the compiler told me I needed to have all these bounds on my function:

use lettre::{AsyncTransport, Message};

pub async fn send_alert<T>(mailer: &T, subject: &str, body: &str) -> anyhow::Result<()>
where
    T: AsyncTransport + Send + Sync,
    <T as AsyncTransport>::Error: 'static + Send + Sync,
    <T as AsyncTransport>::Error: std::error::Error,
{
    let email = Message::builder()

Describe the solution you'd like
I feel AsyncTransport should specify these bounds itself. At least for errors, requiring Send + Sync seems pretty reasonable.

Describe alternatives you've considered
Likely making my own trait if I need more mailing funcs.

Additional context
#356 & this is on 0.10.0-rc.6