Are you aware of any research that measures the combined effect of direct mail and email marketing a business?

Looking for metrics to quantify the lift a direct mail marketing campaign can achieve when augmented with an email marketing campaign.

I'm looking for existing research that's available, not how to conduct my own.

You have to set it up so a holdout cell of records gets only direct mail. The quantity depends on your overall universe. The other test cells obviously have both direct mail and email. You can then measure the impact in response rates by looking at a cost per lead or other cost-per metric. You need to do it with costs because obviously it's more money to do both direct mail and email.

Check out emailstatcenter.com. I'm not sure if the site will have information on direct mail as well, however, it is a start.