email option in concurrent request's delivery options pop up suddenly stopped working

12.2.9 . We have a scheduled concurrent request that runs every day. In the Delivery options pop up's email tab, we provide from email id, subject and to/cc email address. This was working until Friday and suddenly stopped to send emails.

What is the starting point to troubleshoot this? We don't use email through concurrent request's delivery options on any other request and so we don't have a reference point to check where it works and where it does not work.

Tried on another request and none sent for that request too. So, does not seem to be a concurrent request specific .