Answer
By default, the Order Report/print dialog's email uses the logged-in user's own email address as defined on their Create User record as the sender address; if that specific user's record doesn't have an email address populated, it falls back to whatever email address is configured on the Mail Sender connector instead. If sending fails specifically for one user (or a class of users) but works for IFSAPP, check whether those users actually have an email address populated on their own user record before assuming it's a permissions or event-configuration issue.