440 Login Time-out
Meaning
A Microsoft IIS/Exchange specific status indicating that the user's session has expired or is not authenticated, essentially a login timeout.
When to Use
When a client’s session or token has expired on a Microsoft server (like an Outlook Web Access session), and the server directs the client to reauthenticate. This is not an official HTTP status but used by some Microsoft products.
Example
In some versions of Microsoft Exchange or SharePoint, if a user's session cookie is expired, the server may return 440 Login Time-out, prompting the client to redirect to a login page.
When Not to Use
Do not use in non-Microsoft contexts. This is not part of the official HTTP standard. For standard applications, a 401 or 403 with a login page would be used instead.
Source
Unofficial (Microsoft IIS/Exchange)