As we all know, Facebook applications send email to its user via the notifications.sendEmail (for details of theAPI, see here).
While Facebook is in the progress of deprecating this API, Facebook has recently placed even more restrictions on this API can be used.
First of all, Facebook has limited the rate of invoking this API. This reduces the impact of having an application that intentionally sending thousands of spam mails out in a short period of time.
Besides, Facebook will now perform checking on the links that appear in the email content. Facebook will remove the link if it is not pointing to any of Facebook application’s canvas page.
These measures definitely reduce the number of spam mails Facebook users receive from time to time. However, this may affect many applications that uses this feature “correctly”. And most of all, it seems to me that these changes are released without any announcements.