• Fixed typo 'marking' instead of 'marketing' in case-insensitive mailbox name list.
• When DNS-based deliverability checks fail, in some cases exceptions are now thrown with raise ... from for better nested exception tracking.
• Fixed tests to work when no local resolver can be configured.
• This project is now licensed under the Unlicense (instead of CC0).
• Minor improvements to tests.
• Minor improvements to code style.