Onionmail as introduced there: http://onionmail.info/rulez.html (English language home page http://en.onionmail.info/ ) is a system that aims at providing highly secure private and anonymous email, comprised of federated Tor hidden secure email servers. Pop3 & smtp, with STARTTLS extensions, are used above the Tor transport, and user "pop" maiboxes are encrypted at rest using various keys (see description on their site). The server software is open and auditable. Anyone in capacity of running an "onion" could host an "onionmail" server (including "bad guys"of course.)
Having set up a couple user accounts as well as a couple friends', and played with the system for a few days, I could not fault its principle nor operation (availability appears to be pretty good).
Hence I am surprised that Onionmail has received little to no mention and/or attention, here on Stackexchange or on the Torproject blog even.
My question : assuming (1) onionmail messages always end-to-end encrypted using pgp/gpg for instance (in order to prevent the recipient's onionmail operator, or an evil maid, from snooping), and (2) all messages are exchanged between onionmail addresses (thus not using onionmail's additionnal facilities to send to/ receive from the clearnet SMTP network), then...
is onionmail truly anonymous (even unobservable) and its contents private (from prying eyes) by all but, perhaps, a formidable all-seeing (nation state) adversary - if even that ?
Obviously in the frame of this question it is also to be assumed that the end users are not already under targetted by surveillance, or compromised by trojans and the like.