Domain email, often called business email, uses your website address after the @. It is usually provided through a service such as Google Workspace or Microsoft 365, and connected to your domain through DNS settings.
You can still read it in the Gmail or Outlook app on your phone. The difference is the address customers see, and that you control the accounts if staff come and go.
Setting it up properly includes records called SPF, DKIM and DMARC, which prove your emails are genuinely from you. Without them, your quotes are more likely to land in spam.
A quote from info@smithplumbing.co.uk looks more established than one from smithplumbing1976@hotmail.com, and some customers and commercial clients take it more seriously. Properly configured domain email also helps your messages reach the inbox rather than the junk folder.