Workflow Generates the error 'The author internal id or email must match an employee' (SSS_AUTHOR_MUST_BE_EMPLOYEE)

There are a number of scenarios that can cause the error 'The author internal id or email must match an employee'  (SSS_AUTHOR_MUST_BE_EMPLOYEE) when a workflow makes use of the 'send email' action.

Two likely causes of this error are that the 'Send Email' action is referencing an empty employee field for use as the email sender, or the workflow is using the current user as the sender and the workflow is being initiated by a script.

Alternate solutions for handling these two situation are as follows:

1) The referenced employee field of the sender is blank. (Sales Rep is used as a sample employee field in this scenario)

a) In your workflow, create two duplicate 'Send Email' actions.
b) In Send Email Action 1
    - Set your Condition to Sales Rep Is Not Empty
    - Set 'From Field Record' to Current Record
    - Set 'Field' to Sales Rep
c) In Send Email Action 2
    - Set your Condition to Sales Rep Is Empty
    - Set 'Specific Sender' to a default employee to use as the email sender

2) The referenced employee is the current user and the workflow is being triggered by a script.

a) In your workflow, create two duplicate 'Send Email' actions.
b) In Send Email Action 1
    - Set your Condition to the formula {user.id}!=-4
    - Set 'From Field Record' to Current Record
    - Set 'Field' to User
c) In Send Email Action 2
    - Set your Condition to the formula {user.id}=-4
    - Set 'Specific Sender' to a default employee to use as the email sender

Note: -4 is the internal id of '-System-' which is the equivalent of Current User when a workflow is triggered by a script.