When your database was created, the first user's email address always defaults to the account executive that worked with you on signing your contract. The individual at your organization who signed the contract will have that first user account's credentials sent to them. The first time they log in, it is expected that they change this to their own email address instead. If they did not do this the first time they signed in, it can be updated by clicking Welcome, user...>My User > Personas > and update the Email field.
If you've granted support access to log in and test issues in your database, there is a user that may have a Blackbaud or eTapestry email address. This is normal, and the user will only have access if you enable it on the Rights page of the account.
If you've worked with a consultant on a Professional Services project, they may have a user account in your database in order to assist with the project. If you are no longer working with that consultant, you can delete the user account if needed, but do not alter the user account during an active consulting project.