smtp

my docker is config to use ipv6, you force ipv4 using 0.0.0.0 instead of localhost.

Less code to maintain is always a good idea.
https://pkg.go.dev/golang.org/x/time/rate

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