Question:

Why does the Custom From Email Address need to be a valid email address?

Note: The Custom From Email Address is an optional premium feature. For more information on this feature and how to request that it be enabled for your site, please see Knowledge Base Article 000004327 - Custom E-Mail Notification Sender Information.

Answer:

The value specified for the Custom From E-Mail Address also acts as the address where bounce-back and out-of-office e-mails are received from notifications. The address needs to be a valid email address so that the bounce back emails have somewhere to go.