From iA wiki
As e-mail is one of the most ubiquious tools available on the Internet, it makes sense that any anonymous communication would take place through e-mail.
Cautions about Anonymous E-mail
1. Not a reliable method to contact your recipient. Some mail servers block anonymous connections and sometimes the service is not available or does not function. You may wish to ask the person to leave notice in a public place that they received it.
2. Should never be relied upon to be absolutely anonymous. A dedicated source (such as a government or major company) could likely track down anonymous e-mail. Most anonymous resources will cave under legal pressure as well.
- Don't do anything illegal - if something is even potentially illegal, use cryptography to avoid hurting companies that provide anonymous resources.
- Don't send threats or veiled threats - many anonymous resources are very eager to help law enforcement track you down as you'll be screwing it up for the rest of us.
- MixMinion - A unix tool (Linux, MacOsX, etc) for sending and receiving highly anonymous email. <-- !! WARNING !! Type III remailers are still in test! They shouldn't be used!
- Jack B. Nymble - (Windows) - An easy-to-use program for anonymously sending and receiving e-mail (using remailers & nymservers).
- AnonProxy.net Anonymous proxy web browsing at http://www.anonproxy.net.
- Quicksilver (Windows) A program like Jack B. Nymble.
- Mixmaster (Linux & ports) The REAL Type II remailer client/server. With this great program you can run your own remailer or post (usenet too) your mails.
- Onlamp article on Privacy and Anonymity in Email
- An anon email site in the UK
- An anonymous email site with an optional fee-based charge to allow anonymous replies
- Cyberatlantis.com's Anonymous E-mail and their links section.
- TheFreeSite's anonymous freebies
- http://retroshare.sf.net serverless Instant Messenger with Encryption and File Sharing and Trusted Secure Email service for offline Messaging to trusted friends
- Riot Site of an italian Type I and Type II remailer. You could post to email or usenet by https web form also.
- Anonymous e-mail from Sealand Anonymous Proxy Email from Sealand/Havenco Servers.
- WillSelfDestruct.com Anonymous self-destructing (one time seeing only) webmail.