Interact with external users over email.

Your customers can send email to your dedicated Colabolo email address, say support@your-team.com. Colabolo shall automatically create a new issue from the email and assign it to the project leader.
When you or your team member comments on it, you have the option to send it as an email reply back to the customer. If the customer replies, that email is added as a comment to the existing issue. This is perfect for running a help desk.
