mail: Fix charset encoding issue

This PR includes:
- An important fix to charset encoding of from address
- Documentation and examples cleanup
- PEP8 fixes
- Warning on insecure access
- Strict parameter typing
- More modern interface (using lists rather than comma, space or pipe-delimited strings)
- Warn on failure to send mail to some recipients
  ```
   [WARNING]: Failed to send mail to 'foobar': 550 5.1.1 <foobar>:
Recipient address rejected: User unknown in local recipient table
  ```
- Warn on failure to parse some headers
  ```
  [WARNING]: Skipping header 'Foobar', unable to parse
  ```
- Return failed recipients as return value
- Changed default encoding to utf-8