Yes, great. That did it.
Here step by step:
First create a file named /etc/postfix/smtp_sasl_passwd
@<senderdomain1.tld> <username_relay1>:<password1>
@<senderdomain2.tld> <username_relay2>:<password2>
Then create a file named /etc/postfix/sender_relay
@<senderdomain1.tld> <relay1.tld>
@<senderdomain2.tld> <relay2.tld>
Then locate /etc/postfix/main.cf.local and enter:
smtp_sasl_auth_enable = yes
smtp_sasl_security_options = noanonymous
smtp_sender_dependent_authentication = yes
smtp_sasl_password_maps = hash:/etc/postfix/smtp_sasl_passwd
sender_dependent_relayhost_maps = hash:/etc/postfix/sender_relay
Finally do the following commands:
ucr commit /etc/postfix/main.cf
postmap /etc/postfix/smtp_sasl_passwd
postmap /etc/postfix/sender_relay
systemctl restart postfix
Worked immediately. Thanks