smtp is_deliverable is always false

why can smtp.is_deliverable  field always be set to false? even for my own mail

smtp response always look like this
```rust
smtp: Ok(
            SmtpDetails {
                can_connect_smtp: true,
                has_full_inbox: false,
                is_catch_all: false,
                is_deliverable: false,
                is_disabled: false,
            },
        ),
```