I've got a EC2 / centOS vps, it's got postfix / sendmail for pushing out mail.
Now, the domain it self is set up on Google DNS, with it's mail records pointing to gmail servers. Gmail is used for primary email / smtp.
Back to the EC2, the ec2 does a weekly email... you can see where I'm going with this; Gmail / Yahoo / etc think the EC2 is a spam server, the from / reply to email is the correct domain, but the server's address / ip doesn't match the domain's MX record.
What's the best way to associate this?