I have email Full Service and I want to use it to send emails from postfix. I followed the tutorial but the instructions seems to be setup for store & forward. Can please share the differences?

Example: /etc/postfix/sasl_passwd

Tutorial:
[relay.dynu.com]:2525 relay@relay.yourdomain.com:YOURSMTPPASSWORD

I think:
[smtp.dynu.com]:2525 relay@relay.yourdomain.com:YOURSMTPPASSWORD

Am I correct? Are there any other difference?
Lastly, where do I set my YOURSMTPPASSWORD

I resolved the issue by using one of the user accounts. Now I'm receiving the error below

554 Invalid From Address. (in reply to end of DATA command))

Make sure the sender email address domain name matches the domain name you have for the full service email.