Hey there Harley - integrating with other email tools is a great suggestion and something I would definitely be interested in hearing more about. Are you primarily using Mailchimp for your email campaigns? Any other tools out there of interest to you, (or any other CallRail users out there) such as SendGrid, Constant Contact, or Emma?
In the meantime, I do have a workaround for you. You can use utm_campaign in your URL to designate email campaigns, i.e. www.yourbusiness.com/?utm_campaign=mayemailnewsletter or whatever works for your organization's reporting. CallRail will automatically parse that campaign data from the URL if you're using keyword-level tracking, and pull that into the call log, the call details page, and any email alerts you may have setup. You can then report via the campaign filter to effectively determine how many calls you're generating from email.