Filter binary attachments?
: This is, for the most part, a matter of personal philosophy. Some binary attachments, such as videos, sound files, etc., are good; others, such as malicious executables, are bad. The question you as a remailer operator face is whether you want to attempt to police your service, at the risk of being overly restrictive, or run a free and open service, at the risk of letting harm be done through you. This question is moot if you run a middleman-only remailer, however, as you should then always allow binary attachments, so as not to infringe on those remailers who have chosen to allow them.
