Sue, I had the same problem with forwarded work email. The message would have an attachment, Mime attachment - 16kb, for example, you click on that, and it says did not load from the server.
I was able to fix this by opening mail in a browser that supports flash - I am using Puffin, but there are others. I can use the mail app for other accounts, just not my work account where mail is forwarded, possibly there is security settings through our IT center that prevents it from working, I don't know. I did try opening the mail in Safari, that didn't work either.
Good luck, I hope this helps.
Drea1