Email authentication - username vs. username@domain

Hello, I am in the process of migrating an old mailserver over to an account on cPanel. On the old mailserver, users had their own system (Linux) accounts, and therefore could authenticate with just the username, no domain - and most do authenticate that way - just username. On cPanel, where this mail is being migrated to, I have created a parent account for all of these users, example.com. I then have created email accounts under the example.com account for these users, e.g. user1@example.com. When trying to setup an Outlook profile, it does not accept just the username when authenticating, but instead I am required to input the full user1@example.com address. Is it in anyway possible configure it (WHM, this specific cPanel account, etc.) to allow users to auth with only their username? It would make the migration process much less of a headache. :) Please let me know if more information is required. Thanks!