[MM3-users] Receiving 8bit-clean mail

A subscriber to a mailing list I administer has been sending 8bit UTF-8 encoded mail, with no further quoted-printable / base64 / etc. encoding. This is handled fine by my MTA and MUA, but Mailman seems to have trouble with it. When mail is sent out from Mailman, the « character is replaced by ??.

Is this a bug, or is there just a configuration option or something I need to set? I couldn't find anything in the documentation.