Oh... This may be because of the following awkward situation:
Several users have more than one account from the same email address. This isn't just for idiosyncratic reasons; some teachers create a bunch of accounts for their class, using their own email address, so they can handle password change requests for their students (which always happen, and sometimes the school won't accept incoming mail to student email addresses).
Snap! itself is perfectly happy with that situation, but the forum software isn't.
So we solved that problem by creating a unique email address for each account; if you tell us foo@bar we make it foo+snapNNNN@bar where NNNN is a unique number. This works for many email providers (in particular it works for Google, which provides email services to many schools), but not for others (in particular not for Yahoo, apparently).
So email from the forum to some people bounces. This is on the medium-urgent list of problems to solve; not urgent because student/teacher accounts still aren't done, and because we really don't send you email ordinarily, but medium-urgent because some people like to get forum digests by email.
I didn't make myself clear... It's because of people with multiple accounts that we implemented this unique-email (mis?)feature, but the email modification applies to everyone.