Use below command:
echo "Subject: sendmail test" | sendmail -v firstname.lastname@example.org
1.) Create a file called mail.txt (or anything you like) in ~/mail.txt with vim or nano or your preferred text editor
2.) Paste the following content to it, but of course adjusting the email addresses, as those are just sendmail command examples:
Subject: sendmail test two
And here goes the e-mail body, test test test..
3.) At last we send the e-mail template we just created with:
sendmail -vt < ~/mail.txt