To use Office 365 for custom email with your domain, follow the general steps in Microsoft's documentation for help with setup. You'll need to edit records in Advanced DNS Settings to complete the process.
Note: Setting up Office 365 for custom email falls outside the scope of Squarespace support.
Get a new email address with G Suite by Google
If you don't already have email set up, we recommend trying our partnership with G Suite by Google instead. With our partnership with G Suite, you can create a custom email address for your domain, billed directly through Squarespace. You'll also get access to other G Suite features like Google Calendar and Google Drive.