A week after installing the new operating system named OS X El Capitan on my Apple MacBook Pro, I tried to send an email using my Outlook email address via the Mac Mail app. Unfortunately, I got an error message stating “The sender address firstname.lastname@example.org was rejected by the server smtp-mail.outlook.com”. I would like to share how I fixed this issue with my fellow readers and victims.
Follow the procedure given below :
- Open Mac Mail app.
- Click on “Mail” at the top and then click on “Preferences”
- Click on “Accounts” and then select the Outlook account.
- Under “Account Information” tab, locate “Outgoing Mail Server (SMTP)”. Click on the drop-down menu and select “Edit SMTP server list”.
- Make sure the information given below tallies with the ones on the pop-up window 
Under “Account Information” tab :
- Server name: smtp-mail.outlook.com
Under “Advanced” tab :
- Port: 25 (or 587 if 25 is blocked)
- Authentication: External (TLS client Certificate)
- User Name: *Your full email address*
- Password: *Your Outlook password*
Finally, click on “OK” and then close the window. In the next pop-up window, click on “Save”.
Hooray ! I was able to send emails via the Mac Mail app again. Hope this helps.