Last month I got six “DNS timeout” messages, similar to one I’ve pasted below these also mean that I get six “Reverse DNS is set correctly at ISP.” changes as well.
Your box's reverse DNS is currently [timeout] (IPv4) and box.example.com (IPv6), but it should be box.example.com.
I had the same problem with my box a few months back. The problem disappeared when I added a AAAA record for my box’s IPv6 address (I had only a A record for my IPv4 address before). I was never able to pinpoint this change to the solution but I recommend giving it a try to see if the timeout emails stop.