Here you go:
https://openacs.org/doc/current/webmail/
Caveat emptor: it is likely VERY stale (do not know of anyone using it right now). It is an Arsdigita package so it is likely that it needs some serious petting to get up and running.
IIRC Nima tried to get it to work a while back, so he might be able to comment.
Summary: it is not (yet) part of .LRN (it needs a rich uncle or someone with some serious cahones to clean it up... you sticking around after what I would call a suboptimal install experience shows that you might have the latter so you might want to take a look at it).
