365 Outlook send mail as another Mailbox

1. Create a new e-mail. You don't have to fill it out.

Depending whether you use the Classic or Simplified Ribbon, the next steps might differ.

Simplified ribbon;

1. Click on Options, then the 3 dotted button.
   

2. You'll see a section Show fields, press on From.
   

Classic ribbon;

1. Click on Options.
2. Then on From.
   

A new field has been added, titled From.

Click it to change the sender of the mail.

Do note, you still need to have permissies to send from said mailbox or emailaddress.