We have some webfonts we define in our stylesheet, but we want to have these fonts available in emails that our clients receive. If a user has been to the site they have seen those webfonts, the question is if those fonts are now available to other applications on their machine or other websites that declare the font in font-family but do not provide download of the webfont?
Web fonts downloaded from the web are stored in the browser's cache. At any time, those fonts can be removed for other data or because it expired. Keep that in mind.
Email clients, within the browser, are web applications just like any web page and have access to those web fonts but, if they are not in the cache, they will be downloaded.
Email clients that are not part of the browser will not have access to the browser cache.