Few months ago I used to have the usual infamous message :
"Truncated! This message exceeded the Maximum Message Size set in Account Settings"
The usual solution is to increase the Maximum message size in the account settings.
Or even, uncheck the function.
I have three computers and NONE of these solutions worked.
And god knows this is the ubiquitous suggestion described everywhere!
So if you are in the same situation as I was, just try this:
Just uncheck "Fetch headers only" in Tools / Account settings / Server settings and that's it!