A cypherpunk anonymous remailer also known as a Type I remailer is a type of anonymous remailer that receives messages encrypted with PGP or GPG, follows predetermined instructions, strips any identifying information, then forwards these messages to the desired recipient.

This type of remailer is largely considered outdated and comes with flaws fixed in later generations which are called type II and type III. One such flaw is that the messages sent to these remailers are all sent in the order they were received. This allows traffic analysis to determine who sent the message. Before Mixmaster or type II remailers solved this issue by sending mail all at the same time or in waves based on time received, people would try to fix this issue by chaining type one remailers together to further obfuscate the sender. 

When type II remailers came into existence, they built upon the technology of type I remailers. This made type I remailers mostly obsolete. However there are still websites and systems that rely on this general idea of layered encryption and identity obfuscation.

Uses
While they are mostly considered obsolete due to mixmaster being the most common and easily accessible remailer type, cypherpunk remailers can still be applicable in niche applications for those who have no other accessible options. For example, sites that are censored or blocked by governments can use remailers to circumvent the censorship. Cypherpunk remailers require the least amount of work and resources to run, and can therefore be a good solution for those with little assets or time to spare.

Tools and resources for use of remailers
The easiest type to run is a type one remailer however you can optimize and upgrade as you learn more. There are numerous tools on GitHub that can assist a person with using remailers such as remail.sh. This tool allows the user to easily chain mixmaster remailers together, and cypherpunk-cli allows the user to chain multiple type one remailers.

See also

 Anonymity
 Anonymous P2P
 Anonymous remailer
 Mixmaster anonymous remailer (Type II)
 Mixminion (Type III)
 Onion routing
 Tor (network)
 Pseudonymous remailer (a.k.a. nim servers)
 Pen net remailer
 Data privacy
 Traffic analysis

Tutorials and examples 
About.com: Send Email Anonymously — Chaining Remailers with PGP 
Feraga.com: How to use a Type I Anonymous Remailer (link not active 12 May 2010, see archive version)

Notes

 The additional headers used in this context are referred to as 'pseudo-headers' because they are not included in the RFC 822 headers specification for email.
 Messages sent to Cypher punk remailers can be layered, meaning they pass through multiple Cypher punk remailers in order to minimize the chances of identifying the sender.
 Some Cypher punk remailers also function as Mixmaster anonymous remailers, enabling them to divide long Cypher punk messages into Mixmaster packets and forward them to the next remailer if it supports Mixmaster functionality.
 Many users of Cypher punk remailers may choose to repeat steps 1-4 in order to add additional layers of protection to their messages, routing them through multiple remailers for enhanced privacy and security.

Further reading

 Email Security, Bruce Schneier ()
 Computer Privacy Handbook, Andre Bacardi ()

References

Internet Protocol based network software
Anonymity networks
Routing
Network architecture