How to find and evaluate anonymizing remailers?

Following this post:

Anonymized posting mechanism for members-only-post mailing lists

I’m interested in an anonymous remailer which makes emails appear to originate in a fixed address (which would be allowed to post to a certain mailing list). Now, I’ve read the Wikipedia article on these remailers; and it seems like what I need is the use of a pseudonym + removal of "Received:" headers up to the remailer.

My question is, how can I find (free) remailers which offer this feature? And is there a way to verify they really offer it other than by starting to use them? So far, I’ve found mostly dead links and/or opaque systems with a web form.

Notes: I’m not interested into ultra-secure government-wiretapping-resistant remailer. Just something that a typical knowledgeable person would not be able to trace back to the real author. What I do need to be able to do is send (not-huge) attachments.