I have created 2 admin users and they are both subscribed to "Send To Publish" notifications for a content page. When an editor updates the content of this page and hits "Save and send for approval" button, then only 1 admin(user with smaller userId) receives notification e-mail and the other doesn't. I have tried this with multiple smtp servers and the same result.
Umbraco version 7.5.7 assembly: 1.0.6219.11990
Heads up! This is a symptom of a general bug in the notifications. This can happen to any notification in case someone only signs up for one specific notification and others have signed up for the same notification (not necessarily exclusively).
It's caused by the i++ in this line should have been placed inside the body of while instead of as part of the clause.
I'll submit a PR shortly.