1. A method of facilitating secure commenting on content items among collaborators via external messaging applications in a collaborative cloud-based environment, the method comprising: sending a notification associated with a content item to a collaborator via an external messaging application, wherein the notification includes a message and a message authentication code, the message authentication code generated as a function of the message and to be used to verify the integrity of the message when a response to the notification is received, wherein the message includes one or more of a user identifier associated with the collaborator, an item identifier associated with the content item, or a timestamp; receiving a response to the notification from the collaborator via the external messaging application, wherein the response includes a text-based comment associated with the content item, the message and the message authentication code; and determining a validity of the response by verifying the integrity of the message using the message authentication code by: extracting and decoding a response header from the response, the response header including the message and the message authentication code, descrambling the response header using an encryption key to identify the message and the message authentication code, extracting the message and generating a new message authentication code based on the message using an integrity key, and comparing the message authentication code to the new message authentication code to verify the integrity of the message.