I don't know if this is helpful, but our company has had a heck of a time receiving emails when the notification is coming from a person who was not a user in Workfront prior to being added to the approval workflow. The reason is that, if you add them via the Proof tool, it creates a new user where the email address you entered is their first name in Workfront.
So, let's say you included the email address "AmyHo4@company.com" in the approval workflow and this was not associated with a current Workfront user. Instead of the newly created Workfront user being named "Amy Ho4", it would be named "AmyHo4@company.com" with no last name. Then, when a notification email is sent, the "From" field in Outlook/your email provider will read "AmyHo4@company.com <notification@my.workfront.com>". With our security setup, this was flagged as a spoofing attempt and we have actually not found a solution for this with Outlook/security. Instead, we have to make sure that we've added any necessary reviewers as Workfront users before including them in the approval workflow.
I hope this helps. If it doesn't, hopefully it will help someone else reading this post!
