[Terry]

I also have your problem, I subscribed to a news letter and missed 2 replies it was only when I put their name in my address book that the 3rd email was in my in box, it seems emails with links get discarded.
My spam settings are not very high.

I wrote to 8 people in the UK and only got 1 reply which I thought was strange, but my email had a website link so of they hit reply fastmail may have rejected that link.

My wife has also noticed a drop in volume of emails in her mail box.

May have been my sieve script, I have deleted most of it now, perhaps Fastmail changed their rules so it in turn changed mine....who knows.