There was a race condition internal to the go-smtp library.
In order to fix it, a version bump was necessary.
However, this significantly changed the library interface.
We need to unlock the user keyring anyway to unlock the address keyring,
so we should just return it instead of re-unlocking the user keyring
when sending a message.
When an IMAP client appends a message to a mailbox, it can specify
which flags it wants the appended message to have. We need to handle
these in a proton-specific way; not-seen messages need to be imported
with the Unread bool set to true, and flagged messages need to
additionally be imported with the Starred label.
