Thanks for the input, agree Windows may not be the problem, but that is one bit of software that is always updating. I also agree with your comment about Google search in fact that was how I found this site.
The frustration is that it worked fine for a number of years.
Regret I don't have an answer, but I have the same problem with my Brother MFC-J6530DW.
Same symptoms for years worked perfectly, now rejects envelopes with message paper size sent by PC is A4.
Seems to suggest a Windows update or something has caused problem.
I hope like you that some expert...