basic.rs failing to compile without async-std

Without the async-smtp dependency the compilation of the example/basic.rs failed with:
```
let messages: Vec<_> = messages_stream.collect::>().await?;
| ^^^^^^^ method cannot be called on `impl async_std::stream::Stream+std::marker::Send` due to unsatisfied trait bounds
```

Start with example/basic.rs and reproduce the compilation failure when the async-smtp dependency is absent. Trace the messages_stream collect call and its trait bounds; done means the example compiles without async-smtp.

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