Whenever a comment is posted on the discussion page of a project or the discussion within a task, an email is sent out to everyone who is part of the project and has notifications turned on.
When people receive this email they have 2 options:
They can click the link at the bottom of the email to login and reply.
They can reply directly by email.
Note: If they choose to reply directly by email then they must use the same email address that they use to login to Project.co. This is so that their email can be attached to their user on the project.
When someone replies to a comment by email their reply will be posted on the discussion feed in the relevant project/task.
Note: Attachments such as images and files that are sent by email won't be posted to the project. If you need to upload a file along with your comment then you need to login and reply directly on the project/task.
The "reply to" email is specific to that project/task. So you could also use that email address to email in to the project/task at any point.