Error 'Username does not have access to this record and will not be updated' When Mentioning Users in Chatter Posts

This error occurs because the person being @mentioned does not have record-level access to the record associated with the Chatter post. Salesforce Chatter requires that mentioned users have access to the underlying record. Follow the example below to understand the cause and resolution.


For example:

AW Computing has its Account object set to Public Read/Write, and its Case object set to Private. Jim owns an Account named ACME Testing. Since Sharing Settings are Public, Jane is able to see the ACME Testing account that Jim owns.

From the Chatter feed on ACME Testing, Jim uses the New Case action to create a new case and assigns himself as the owner. When Jim reviews the Chatter feed for ACME Testing, he sees a new post for the new Case that he just created. Jim attempts to mention Jane on the post to let her know that this new case has been created, but he receives an error "Jane Doe doesn't have access and won't see this update." 

Jim decides to save the post anyway but notices that Jane's name is grayed out after it saves. Jane lets him know that she never received an email about the post.

Even though it is in the feed for the Account record that Jane can see, the Chatter comment Jim is making is directly related to the Case that he just created. Since the organization's Sharing Settings are set to Private on Case, and Jim is the owner, Jane cannot see the Case and will not be notified of the mention to her. 

If a Partner or Community user is @mentioned, in addition to having record access, "All with Access" should be selected.

If the object in question is a custom object, the object status should be 'Deployed'.