Of course there is always the discussion of "do we want to confirm that
an account with this name exists?" Moin wikis just say "If this account
exists an email was sent.".

This sometimes is really annoying, so I would prefer to only have this
as an option (if at all), not the default, as the default behaviour of
roundup is a public tracker, even if we mostly use non-public trackers.

To make the obfuscation yield more privacy, I suggest obfuscating more,
especially in the username part, as this often includes the real name of
people.

What about replacing everything with dots, except for the first
character of the localpart, the @ and the last character of the domain
(and the dots :)), e.g.:

user@example.com -> u...@..........m
user@example.net -> u...@..........t
firstname@example.com -> f........@..........m ->
firstname.lastname@ntvtn.de -> f.................@.......e