You can send email anywhere in LAC where you can write code. A typical place would be as an event on insert, say if you insert a record for a user registration. You will of course need an email server to communicate with, or some sort of email service like Amazon Simple Email Service. If you're just using SMTP, you might consider using Apache Commons Email, which makes it easy. Just add the jar to your classpath (e.g. lib directory for Tomcat), and you can send an email with:
var SimpleEmail = Java.type("org.apache.commons.mail.SimpleEmail");
var email = new SimpleEmail();
email.setHostName("smtp.acme.com"); // Your SMTP server
email.setSmtpPort(25);
email.setFrom("jdoe@foo.com");
email.setSubject("Test Mail");
email.setMsg("This is a test mail ... :-)");
email.addTo("bob@acme.com");
email.send();
Full documentation is here: http://commons.apache.org/proper/commons-email/userguide.html