How to Change the Default Email Composer in Salesforce Lightning Experience

To set your preferred default email composer in Salesforce Lightning Experience:

1. Click your profile icon in the top-right corner of Salesforce.
2. Click Settings.
3. Navigate to Email | Email Settings.
4. Under When you click an email address to compose an email, which email editor do you want to use?, select your preferred option:
   1. Salesforce Email Composer — Use this if you want to compose emails within Salesforce.
   2. Default Email Application — Use this if you prefer to use your computer's email client (such as Outlook or Apple Mail).
5. Click Save.
   
   

Note: If the Email Settings option is not visible in your Personal Settings, refer to the related Knowledge Article for troubleshooting steps.
https://help.salesforce.com/s/articleView?id=000380356&type=1

Testing Your Setting
To verify that your selection is working correctly:

1. Navigate to any Salesforce record that contains an email address field (for example, a Contact record).
2. Click the email address in the email field.
3. Confirm that the correct email application opens based on your selection.

Setting Your Default Email Application on Your Computer
If you selected Default Email Application in Salesforce but your computer is not opening the correct email client, verify your computer's default email app:

On Windows: Click the Start button and type Default Apps in the search bar. Under the Email section, confirm or change your default email application.

On Mac: Open your current email client (e.g., Apple Mail), go to Mail | Preferences | General, and set the Default email reader to your preferred application. For more guidance, refer to Apple's support documentation on setting a default email reader.

Additional Mac Troubleshooting: If you are trying to set Microsoft Outlook as your default mail client on a Mac and are experiencing issues, refer to Microsoft's support documentation for your version of Outlook.

In Mac: How to set you default email reader in Mac
Refer to the Apple's support documentation here

Addition information if you are running into issues when trying set Outlook as your default mail client
- Outlook 2016 for Mac cannot be set as the default application in Mac OS X Yosemite
- Set an account as the default in Outlook for Mac