We want to avoid deserialising message contents, because Python's email module
might produce different representation than the MUA sending original message.
The result would be a transformed message, which could mean broken message in
certain conditions.
- Introduce exceptions to be raised upon transient and permanent delivery
failures, as specified by SMTP RFC. Depending on type of failure, return
either 451 or 554 reply code.
- When serialising a message, treat ValueError as a serialisation issue (and
try again to deliver in cleartext).
smtplib.SMTP expects ASCII-only message bodies when message body is provided
as a 'str'. If we pass a 'bytes', we need to choose encoding earlier and we
do this by calling 'as_bytes' on messages with SMTP policy, which takes care
of formatting the body properly.
As a result, ISO-8859-x messages are converted to Quoted Printable and UTF-8
messages are Base64-encoded.
Testing this behaviour is tricky, because we use the same SMTP client to send
test data. For this reason, test code has become a bit ugly, but it does
exactly what we need.
- Polish implementation of mail operations (lacre/mailop.py). Add two
strategies: InlineOpenPGPEncrypt and MimeOpenPGPEncrypt, to support two modes
of OpenPGP encryption.
- In delivery_plan, only use those strategies that actually make sense with
the recipients we'd got.
- Add flag_enabled predicate (lacre/config.py) to make configuration checks
easier / simpler.
- Handle TypeError errors in Advanced Filter, indicating a delivery failure
when they appear.
- Add type hints to some of the functions.
- Add a "mailop" module to define mail operations. Each should inherit from
MailOperation class (which just defines the contract).
- Make lacre.mailgate.delivery_plan always return KeepIntact strategy to have
a daemon that just forwards messages without modifying them.
- Add sample configuration.
- Include daemon configuration in mandatory parameter check.
