To do this, you will need to configure your home mail server's "smart host" in sendmail.
This will be done in your sendmail.cf file. Which on my CentOS server is /etc/mail/sendmail.cf, you may want to use locate or find to figure out where it is on your machine. One in that file, you will see a line that starts with DS. Modify this line to be DS and then the fully qualified domain name of your Postfix server
# "Smart" relay host (may be null)