In an effort to drop the CLI completely in Guice products, we need to support all features of the CLI in the webAdmin alternative.
Currently, one of the features missing are related to "message upload".
As an admin, I should be able to upload a message in any mailbox.
Proposed API is
POST /firstname.lastname@example.org/mailboxes/INBOX -d 'header: A BIG EML\r\nWith a bi body'
You will add it as part of UserMailboxesRoutes class.