1. Outlook for Windows (classic)
Select Mail, and then under the section Compose messages, select Editor Options. 
Once the Editor options page opens, navigate to the Advanced tab. 


For each scenario, choose Keep Source Formatting from the drop-down list:
- Within the same email
- Pasting between emails
- Pasting between emails when style definitions conflict
- Pasting from other programs

Make sure Show Paste Options button when content is pasted remains ticked (this lets you override the default if needed).
2. Outlook for Windows (New Outlook)
Navigate to the top of New Outlook to the settings icon, which is depicted as a gear below, and click it. 

Select the Mail option on the left-hand side, and then select the Compose and reply option. The below window should match what your seeing:

In the Compose and reply section, scroll down to the Cut, copy and paste section and ensure that Keep source formatting is selected for both drop-down menu options. 

3. Administrator options to enforce via Group Policy
If your organisation manages Office settings centrally:- Deploy the Office ADMX templates.
- Navigate to User Configuration ► Administrative Templates ► Microsoft Outlook ► Outlook Options ► Editor Options ► Advanced ► Cut, copy, and paste.
- Pre-set each dropdown to Keep Source Formatting (like seen above but for Group Policy), then deploy the policy or configuration profile.

