In the clickthrough report in the single mailing report section, you can track clicks to Web URLs, but there's no way of tracking links to 'mailto'. It would be great to have this feature, as although you can obviously track how many emails you receive with the bespoke subject line etc for a campaign in the mailbox, these are actual conversions. I'd like to know how many people clicked the mailto link to open an email in the first step of the process.