It is not unlikely that when sending a few hundred emails you will receive errors (full mailbox, incorrect email address...). You will then have to deal with each element.
For example, when shipments sent by EMAIL channel come back with a specific error code, then the application will send again the documents using the MAIL channel automatically.
The error codes are defined by the system administrator. And then in your Service creation you can activate or not the fallback channel.
Note that this applies to all existing channel types.