The fix has been pushed to all tenants and jobs are automatically retrying.

Between 20:00 and 21:40UTC some tenants experienced errors in emails being sent on tenants located in our euwest region. Error Logs will show "Error: Unexpected error: 535 Authentication Credentials Invalid" The issue has been identified and a fix has been pushed to all affected tenants. All email action runs sent will be retried automatically. To find out which stack your tenant resides on, check https://<<tenant.address.com>>/info