I have a working hotmail email address which contains no '.' to the left of and a non-working hotmail email address which also has no '.' to the left of does 'feel like' some kind of routing/protocol rule change that's causing the problem somewhere, but seemingly only related to some hotmail accounts. Interesting thought on the email address format, but I can confirm that's not what's causing my problem. Hopefully if there are other people having the same problem, they'll also raise it so further evidence can be collected.
Hi nicwhit - it seems there's a few of us with the same problem at least! Interesting from Plusnet Support.