[sudosteph]

There are actually 3rd party services that mail companies can use to check how trusted an IP is. So a lot of times, they'll provision an EIP on AWS, run a check against that service, use it if it's clean, and release and try again if it's not.

It does lock you to using EIPs though, which makes it a bit harder to scale up.

[_xnmw]

You might be interested in a provider-agnostic email API I am building to to avoid EIP/provider lock-in and have the security of multiple dedicated IP providers. Check it out: flutemail.com

[Scoundreller]

What's an EIP?

[jffry]

Elastic IP. You can reserve EIPs from among Amazon's pool and allocate them to other AWS resources that need public-facing IPs (like an EC2 machine).