Attempt MX, fallback to relay in case of error only

[cont...@rusanu.com]

Is there a way to configure postfix to use a relay only if direct  
delivery is rejected? The case being a web application that has to  
send notification to subscribers, but some servers reject the  
connection (app IP originates from AWS EC2). I want those cases to be  
retried using a paid SMTP relay like authsmtp.com. However I don't  
want to use the relay for everything since it costs per mail.

Thanks,
~ Remus