Forwarded or Redirected message is garbled

Some emails that I receive are readable in Mail but when they are forwarded, the message is just line after line of garbled text like this: U291bmRzIGdyZWF0IHRvIG1lDQoNCkx1bmNodGltZSBtZ... (etc. it goes on for ages)

I see exactly the same when I view the message as "Raw Source" and I have discovered that this is Base64 encoding however this is clearly being ignored when Mail forwards or redirects the message.

So can anyone suggest a fix for this? Perhaps an Applescript that converts the message correctly before forwarding?

MacBook and iMac G5 Mac OS X (10.4.6)