Set an account as the default in Outlook for Mac

On the Tools menu, click Accounts.

Select the account that you want to make the default account.

 Click on the toggle button in the 'Default Account' field to set the account as default.

Set up your default email reader
Your default mail reader is what's used when you're browsing a web page and share it via email with a friend or colleague or mail it to yourself for further reading. The Mac default is to use the Mail app, but you can change the default to Outlook.

1. Open Finder > Applications.
2. Select Mail.
3. Select Mail > Settings⁠.⁠
4. On the General tab, in Default email reader, select Microsoft Outlook.

On the Tools menu, click Accounts.

Select the account that you want to make the default account.

 On the bottom left, click the three dots and select Set as Default from the list.
Notes

• To send an email message from an account other than your default account, under Inbox, select the account in the list. You can now send email from your non-default email account. When you're finished, under Inbox, click your default account.
  

• When you forward or reply to a message, Outlook uses the account that the original message was sent to. To change this preference, on the Outlook menu, click Settings, and then under Email, click Composing . Clear the Reply and forward using the default e-mail account check box. 
  
  Set up your default email reader
  Your default mail reader is what's used when you're browsing a web page and share it via email with a friend or colleague or mail it to yourself for further reading. The Mac default is to use the Mail app, but you can change the default to Outlook.

  1. Open Finder > Applications.
  2. Select Mail.
  3. Select Mail > Settings⁠.⁠
  4. On the General tab, in Default email reader, select Microsoft Outlook.