OK, it looks like it is specific to certain destination email providers.
I just tried the "Email a friend" feature. I first sent two emails to two GMail addresses. Both failed to arrive - not even appearing in spam.
Then I sent a third email to a non-GMail address (a private .com domain), which arrived in seconds:
Based on this I think that all forum emails are failing to arrive at GMail addresses - perhaps due to blackmagicdesign.com emails not being sent encrypted?
It'd be great if your sysadmin could look at what's required to get forum@blackmagicdesign.com emails to arrive properly again at all providers.