Thanks for looking into it.

However I don't think that what you found applies in our case as it would change the appearance of the entire message whereas what we have is just some parts of the message being affected.

e.g. Here's a screen grab of what's in our "Sent items" folder:
(It is a corporate private message so I have blurred the images so that they are unreadable - you can clearly see the change in formatting though)



And here is a screen grab of what the person we sent it to sees:



For an entirely different message the effect was quite different. The message as received ended up with big gaps in it that weren't in the message as sent.

What was sent:



What was received:



Any other ideas?