Civi emails can be sent from addresses that are On Hold

If a Civi backend user has an email address that is flagged as On Hold, Civi emails sent by that user can still be sent using the On Hold email address, and if the On Hold address is flagged as Primary, new email will default to using that On Hold address.

Seems like it would be useful to not include On Hold addresses in the From address list at all. Or at least give the user a notice that "The selected From address is On Hold. Do you really want to use it?"