Mail in Outbox sent even password is changed ?

Hello,

Tested with iPhone & Outlook 2016 ActiveSync :

1. Configure an ActiveSync account on the device and use it
2. Go away from internet or set up "offline" mode in Outlook
3. Compose new mail, send it. We are not connected, the the mail goes to Outbox and stay in it
4. Change password of account, from admin console or account itself
5. Wait few seconds, minutes... or days !
6. Put device on internet, or go away offline mode from Outlook
7. Mail is sent, even the password has been changed !

How is it possible ? How can i avoid it (i.e. : user leave company,...) ?

Thanks in advance !