Enh: consider storing unrecognisable e-mails in a quarantine area

When importing from mailboxes or using the archiver, there may be the occasional e-mails that fail to parse correctly.

As far as I can tell, these are just dropped at present.

It might be useful to optionally store these in a separate quarantine area where they could be examined and rerouted as necessary.

Start by tracing the mailbox import and archiver paths to find where unparsable e-mails are currently dropped. Define how optional quarantine storage and later rerouting should work, then verify that malformed messages are retained without changing normal imports.

Written by the indexing model from the issue text.