How about send a mail without login? Just try SMTP.
This Program is a example using SMTP to act as a SMTP server.
Have Fun!

After build, run the result program, it will give you the help information.

In this program, I used library libcurl and taked advantage of the feature of version 7.21.4, If you have any problem while compiling, please contact me.

Email : isaymeorg [at] gmail [dot] com
Blog : www.isayme.org [Chinese Simplified]