Click.org

Click.org is a click tracking and link management tool aimed at online marketers and affiliates. It wraps outbound links so that clicks, conversions, and traffic sources can be measured, and adds features such as link rotators, redirects, retargeting pixels, and bot filtering.

Links can be published either on a marketer's own domain or on the shared trk.org short domain the service provides. Seeing those links in an email means click measurement is being handled by Click.org rather than by the sending platform.