Secure relay capability has been added to the Phantom Remailer. This feature allows a masquerading user to send messages through the remailer without needing to use a login and password. This is achieved through the use of a secret token that is known to the users of the remailer (if you choose to share it). This feature is described in the associated Github issue: https://github.com/DerPhantomCoder/remailer/issues/2
Adding anti-SPAM and abuse protection to the Phantom Remailer is the focus of my most recent commits. I created the Phantom Remailer for personal use, but when you release a piece of software to a wider audience it becomes your responsibility to ensure it is secure and robust.
The Phantom Remailer uses the Reply-To header to encode necessary meta information about the sender — while this information was encoded it was not authenticated. The design of the Phantom Remailer does not rely on storing any information about the sender or recipient on the server running the remailer so it was vulnerable to attack by crafting compatible To headers in messages addressed to the remailer.
The Phantom Remailer is a pseudonymous single-blind remailer that runs as a delivery agent on a mail server. It was designed for use with Sendmail and other mail transfer agents that support the Sendmail standard. This remailer implements fully SPF and DKIM compatible mail forwarding with a low Spamassassin score.
The remailer works as a transparent forwarder for incoming emails and an anonymizing remailer for outgoing emails. This means you get the full benefit of Multipart/MIME email messages as an incoming address and a stripped down text/plain experience for outgoing messages, even when your reply contains a Multipart message.