[feat] Support failing faster for unroutable address

[xim]

Describe the feature

Currently, if an email is sent to an unroutable address, the email is deferred (421).

I get that this can be helpful when first configuring a domain, but in a stable setting I'd prefer these to fail with 550 Unrouteable address

Adding a configuration option for this, e.g. adding a config or by allowing aliases to be explicitly set to reject incoming email (so I can add a wildcard reject alias)

Checklist

• [x] I have searched through GitHub issues for similar issues.
• [x] I have completely read through the README and documentation.

[titanism]

@xim thank you for the request! we'll look at adding this – right now we're trying to ship calendars/contacts/newsletter support