Salesforce Inbox Error 'Cannot Log to Salesforce, Datasource is not ACTIVE. Status: NEEDSATTENTION.'

This can be caused by a disconnect between Salesforce Inbox and the user's email account (Gmail, Exchange, or Office 365). Follow the steps below to reconnect the two.

Note: The following steps are performed in the Salesforce Inbox client and not in Salesforce itself.

1. In your email client (e.g. Gmail, Outlook), launch Salesforce Inbox and navigate to the Options tab | Select Settings. 
2. Select Connected Accounts and delete the associated mail account.    
3. Select Outlook Settings and log out of Salesforce Inbox.
4. Log in again and connect the mail account to complete authentication.