Possible language settings in your email client?  Does any character other
than @ appear instead?  What country are you in?  It appears in different
places depending on nationality.  On mine (UK) it's Shift-apostrophe.  In
some countries this will give you a quotation (") symbol instead.  Try
Shift-2 instead.  If this works then your email client is probably set for
the wrong language/keyboard.
....Unless of course you're using Windows Mail for both email and news, in
which case I don't know, as it's obviously working in this thread.
I've just noticed your post was written in Unicode (UTF-8).  It actually
appears in the title bar of this reply, and that's the first time I've seen
it.  This may have a bearing on the subject.  Unfortunately I can't help
here.  Hopefully someone else can guide you through setting the right
character set.