Sending mail from Linux

I have sendmail installed on my Redhat linux(version 5). But I am unable to sendmail. I tried various commands like:

service sendmail status
output:
sendmail (pid 2296 2288) is running...

telnet localhost 25
output:
Connected to localhost.localdomain (127.0.0.1).
220 localhost.localdomain ESMTP Sendmail 8.13.8/8.13.8;

What changes do I need to make inorder to send mail from my Linux? Please tell me the procedure step by step. I am new to Linux.

How *exactly* are you trying to send email? Are you trying to do it via the command line, a script, a desktop program, etc.?

Actually I installed a software(IMT-Mailer) on my linux system. This is used to send multiple emails at a time across the internet. Just we need to start the mailer, set the template, email list and time for sending mails. At that particular time these emails will be sent.

The mailer installed and started successfully. It is also dispatching mails without error. But those mails are not going from the system.As I am new to Linux, I don't know how to configure the sendmail. Also I didn't set domain name. I need step by step procedure for this configuration. I am using Redhat Linux 5.