You need to relay the mails you are sending using your ISPs smtp. Add these two lines in your main.cf in /etc/postfix
broken_sasl_auth_clients = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_always_send_ehlo = yes
relayhost = smtp.yourisp.tld
smtpd_sasl_authenticated_header = yes
smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, check_recipient_access mysql:/etc/postfix/mysql-virtual_recipient.cf, reject_unauth_destination
Then 'create /etc/postfix/sasl_passwd' and add these lines
If your ISPs SMTP server doesn't require authentification than you won't need to add 'smtp_sasl_password_maps .....' line and won't have to create 'sasl_passwd' file.
Hope this helps you.