I haven't been doing cold email...but it's pretty close. I have Google alerts setup for the names of our Open Source products. When a hosting provider is using Webmin, and advertises that fact, I send them an email offering to set them up as a Virtualmin reseller, and listing a few of the benefits they'd get from offering Virtualmin GPL, with the option to upgrade at any time to Virtualmin Professional. I've only done this four times, so far (still refining the reseller experience, so moving slowly), but it's been very effective. At least a 50% success rate (and I wouldn't rule out the other two signing on later, as neither was negative on the idea...just hesitant to change or commit to anything).