By default, workflows typically use one email address as the sender of email alerts in the system. This is called the default workflow user.

(It's possible to change the sender of any email alert to other users in the system, or to an organization-wide email address but by default, its typically the default workflow user)

If you have a staff person leaving the organization, before you can deactivate their salesforce license you'll need to designate someone else to be the default workflow user.