My guess is someone is using images in signatures and doing it badly.

SirWinT said

I am actually replying to a received email. Which was a reply to my original. The original went without trouble.

Thanks that is part of the problem. A slow connection was the other. I got confused by the error message about attaching... Could something be devised which is more accurate? thanks. SirWinT

The error message sounds to me like you have McAfee anti virus installed. Have you?

I have antiviral software, Avast, which does leave a message. But why would that be problematic with a (temporary) slow connection? But yes you are right lots of things get put in the email which I never write. I shall ascribe the original problem to the connection. I appreciate the help. Thanks