Returning to studying Twisted's (a Python networking framework) email implementation, for its larger top-level components...
There's a module datamodelling emails, mainly for the domains & the mail itself.
Parsing of email-alias files & a routing-extension based on that data.
There's datamodelling of mailboxes, abstracting the filesystem.
There's some traits (using Rust's term since I find it clearest) modelling the protocols.
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