Hello I am usin sendmail as an smtp server is there away to start up sendmail ( & start down ) only when I send emails
Sendmail won't work properly like that because the mail server you are trying to send mail to may not be accepting mail so your sendmail server needs to queue it and retry it later.
What is your concern about running sendmail all the time?
One thing you could try would be to edit /etc/mail/submit.mc and change the line:
replacing the 127.0.0.1 with the hostname or IP address of the mail relay you want to use. This would enable you to send mail out without actually needing the main sendmail server running at all, but you would still need to run "sendmail -Ac -q" periodically to flush out any queued mail.
If you make this change, you'll need the sendmail-cf package installed to build the submit.cf file needed by sendmail.