Have a look at this page of
Mail Transfer Agents (MTAs) and their homepages. MTAs are the programs that retrieve and send mail through SMTP. The well-known ones are Sendmail, Postfix, QMail and Exim.
However, speaking as someone who has been spending (too much) time recently trying to get Postfix going, let me tell you that you do not need the aggravation of a full-blown MTA, simply to send your email through your broadband ISP's SMTP server.
If you use Mozilla Mail (a.k.a. Thunderbird), you can simply configure it to use your ISP as the SMTP server, without the need of your own MTA.
If you prefer a console-based email client such as Mutt, you can use a lightweight MTA such as
smtppush.py, which is a simple Python script that sends email through your ISP's smtp server.
smtppush.py is what I am using now; it works great. I copied it into /usr/local/bin and gave it executable permissions. In the ~/.muttrc configuration file, I added the line:
set sendmail="/usr/local/bin/smtppush.py -f username@isp.com -S smtp.isp.com"
where username@isp.com is your email address and smtp.isp.com is your isp's smtp server.
