[SalesForce] Workflow Email Alerts Not Log In Email Related list of Case

In a sandbox the user's email address is changed as part of the refresh process (see guidance here) do that would be why your user cannot receive the email. Update the address to be correct and retry.

If you don't want the email to be sent out at all when the Case was created by a portal user, you could add a constraint to check for the user type of the user that created the case in the workflow rule that triggers the email alert,.

E.g. make it a formula rule with the following formula (checking for the profile name of the user):

NOT(CreatedBy.Profile.Name  =  "High Volume Portal User")

If you have multiple profiles you could add them all, or check for the UserType. E.g. if you have a partner portal/partner community the usertype will be 'PowerPartner':

NOT(ISPICKVAL( CreatedBy.UserType ,"PowerPartner"))

If you want to email to be send out to the other recipients but not to any external Case creator, I'd suggest to

1. create an additional Email field on Case (e.g. "Internal Record Creator Email"),
2. create a additional Workflow Rule to only fill that field when the creator is internal (using the same logic as above), and
3. add that email field to the email recipients in the original workflow.