I'm at a loss as to what avast might be finding suspicious, I don't know what Partial Message Danger! might mean, to me partial could mean either the body of the email or anything for that matter.
What is the sensitivity in The Internet Mail provider and in the Heuristic Tab ?
You could progressively lower the Heuristic sensitivity from High, Medium and Low, etc. You will notice with each lowering of the sensitivity that less options are checked, at each lowering of the sensitivity try sending the email again until hopefully it is successful. So by elimination something in the list of checked options not present that is the one or ones to concentrate on.
You could also set the Heuristic sensitivity to Custom, that allows access to other more detailed options, see image.