Every message has an attachment
From MozillaZine Knowledge Base
Some anti-virus programs scan every new message and add an attachment certifying that it scanned the message and that it didn't find any viruses. If this happens you'll typically see a paper clip icon next to each message in the folder listing and each message will have a "Part 1.2" attachment with text similar to:
AVG is notorious for doing this. If you're using AVG you can disable this feature by
If your email provider added the text you need to login to webmail using a browser and find the appropriate webmail command to disable it.
Note: The anti-virus program won't add the attachment if the message was digitally signed, since the attachment would ruin the signature.