Perhaps you could re-confirm that the email account you wish to send from is as the default in your Outlook account. You can do this by going to File|Account Settings|Account Settings>Email - a check mark will appear next to the default account account.
You might also go to File|Options|Mail>Send messages and make sure that the "Always Use the Default Email to Send Messages" is checked.
Outlook should now use the default email account as the sending email address on a mailmerge.
If that doesn't work, you may need to create a separate Outlook profile with only the account you wish to send from and launch Outlook with that profile. To create such a profile:
1) Open the Windows Control Panel, then go to User Accounts>Mail>Show Profiles.
2) Click 'Add' to create another profile. A dialogue box will apears with panes for you to input input the name & address parameters for the email address to be used. When you've done that, click on 'Next' and follow the prompts.
3) Click on 'Prompt for a profile to be used'.
4) When you have added the other alias into Outlook and click OK.
5) Close and re-start Outlook, choosing the profile you've created for the mailmerge.
See
https://www.howto-outlook.com/faq/newprofile.htm for more details on profile creation.