Okay, gotcha.
You can try something more like this, then, if you want the "From" address to be the same as the original e-mail:
Code:
notify :method "mailto" :options ["xxx@xxx.com", "From", "Orig"] :message "$from$ / [New Email] $subject$ / You've got mail!";
Or, alternatively, you could go with something like this if you want to use a specific "from" address for the notifications:
Code:
notify :method "mailto" :options ["xxx@xxx.com", "From", "Orig"] :message "<me@mydomain.com> / [New Email] $subject$ / You've got mail!";
The key is the slashes in the :message directive — it basically takes the form of "from / subject / body." While FastMail's documentation says that if you don't use slashes, the text should go in the subject line, that doesn't actually appear to be the case, but either way, the three-part slashed option seems to be the only realistic way to do this anyway, as you've discovered.
The "From", "Orig" options after the target e-mail address tell FastMail to expand the "$from$" directive to include the name and the e-mail address. Without it, you'll just get the raw e-mail address.
See
Sieve Notify Extension in FastMail's help pages for more info.