Cargo Features

[dependencies]
async-smtp = { version = "0.9.1", default-features = false, features = ["runtime-async-std", "runtime-tokio"] }
default = runtime-tokio

The runtime-tokio feature is set by default whenever async-smtp is added without default-features = false somewhere in the dependency tree.

runtime-async-std = async-std
runtime-tokio default = tokio

Features from optional dependencies

In crates that don't use the dep: syntax, optional dependencies automatically become Cargo features. These features may have been created by mistake, and this functionality may be removed in the future.

async-std runtime-async-std?
tokio runtime-tokio