If a user replies to a email notification of a task, but the reply comes from an address that Phabricator doesn't know belongs to the user, the reply completely disappears. (This is common for people who have several email addresses directing to e.g. their Gmail, but always reply from one address.) Phabricator should respond to the sender to the effect of "We don't know that you're allowed to comment on this, so your comment wasn't posted."