Mail gibberish

When I receive mail sometimes it gomes in as gibberish. (smashed together numbers)
The From, Subject, To and Date are just a bunch of smashed together numbers. The same is true for the body of some e-mails. If I forward the message to myself it come through fine except for the "from, subject, to and date". They are still smashed together numbers.

I've tried rebuilding mail but nothing works.

Any suggestions