Lesti::Smtp replaces normal Sendmail by Smtp. That allows you to send your emails over the smtp-server of your email-account. This reduces the spam-ranking of your mails.
Where are the configurations?
You can find the configurations under System => Configuration => Advanced => System => Lesti Smtp.
It doesn’t work?
If no email was send, please be sure to enable logging of magento and have a look at /var/log/exception.log. Zend_Mail should write a understandable exception in this log file.
Lesti_Smtp is compatible with Magento CE 1.5, 1.6, 1.6.1, 188.8.131.52, 1.7, 1.8
Lesti::Smtp rewrites the model core/email_template.