some Mail messages appear blank in message field on iMac
I have a new problem with mail on my iMac - some emails have a complete blank message field, and therefore cannot be read. The message appears on my iPhone, and also on my macbook, but in Apple Mail on the iMac, I can't see the content of the messages. This doesn't happen all of the time, or with all messages. A message might come in and be readable, and then the next time I look at it, the preview pane is blank. Opening the email individually doesn't make any difference either.
Any ideas?
Thank you