/etc/postfix/sasl_passwd file in the local postfix directory and add your SMTP credentials.sudo postmap /etc/postfix/sasl_passwd to update the credentials in sasl_passwd. For the username, use the master password from your Brevo account, available in the SMTP and API menu in the top-right. Next, update the main.cf file in Postfix.If the relay does not allow IPv6 connections, set inet_protocols to IPv4 in the main.conf file.
sudo service postfix restart and send a test email with the command below.Note: if you encounter errors, check mailq or the Postfix log file. An authentication error means you entered incorrect credentials, or the changes to the sasl_passwd file were not saved.