Just for prespective, we received the same email yesterday for domestic postage.

So it is not only private sellers as we are a business account and have been for many years.

Upon checking the issue is showing as being "high" on INR's however that actual number was 12 in 1000 sales

All Royal Mail 2nd Letters funnily enough, and also after checking the downloaded eBay report 4 of those 12 sales are showing delivered, so along with all the other tech issues this just maybe another.

That said just make sure your account and all sales are in order is all you can do.