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- Require the `port` field of `SMTPSender` to be non-negative
- Mark `Sender` as `runtime_checkable` and export it
- Gave the `outgoing` command `--section`, `--no-section`, and `--log-level`
options
- Added logging to built-in sender classes
- The `outgoing` command now loads settings from `.env` files and has an
`--env` option