I remember having issues with a corporate email system where base64/uuencoded data would fail to get through with a very rough dependency on size - large files had a smaller chance of getting through but it was clear that there wasn't a hard size limit. Eventually someone twigged that the problem was a "rude word" scanner, and that beyond a certain size you would hit the "scunthorpe" problem, and forbidden words would appear in the ASCII text randomly.