I get that when trying to send through my BES. It may have to do with emails not being allowed to have attachments through the specific version of BES? Just a WAG. The way I work around it is to scroll up and change the mail service from default to tmobile. Then it mails just fine.
If the message is set to "Send Using: Web Client" it won't work. If you change it to "Send Using: " it should work. (obviously replace with your email address.