Using Thunderbird, a free open source email and Usenet client made by Mozilla, and the addon TorBirdy you can now send emails through the tor network, the addon will automatically connect to the tor proxy before login in and out of your email account to send SMTP email and retrieve new POP messages, as long as the chosen email provider does not block tor proxies. For extra security you might want to encrypt the email message, Torbirdy won’t do that for you.
The addon uses SSL/TLS by default for both outgoing and incoming servers setting default server ports, IMAP push email, often used by advertisers, is disabled as it could expose your real IP. The email client itself is cross platform and has versions for Windows, Mac and Linux, forked Thunderbird versions like Ice Dove should also work with the Torbirdy addon, the anonymous live CD Tails has added Torbirdy to their list of future additions.
The developers advice that users do not install any other Thunderbird addon that has not been reviewed by them together with Torbirdy as it could endanger anonymity. Since Thunderbird has a portable version that runs from a USB thumbdrive there is no reason why you should not be able to use Torbirdy as a portable email client to send anonymous messages, it might even get around public computer networks that block port 995 (POP) and 25 (SMTP). There are many free email services offering free SMTP but the sign up normally still has to be done using the web interface, needless to say that you should use tor for that.
Note: This addon is still in development and might not be stable.