Microsoft have annoyingly given the name 'Outlook' to two completely different things. It would help to know which one you're referring to.
Are you using the Outlook email client, which is probably used in most business (as well as by many home users)? That's a specific program for handling email, totally separate to your web browser, which can be used to access mail from any provider. (e.g. you could have an email address ending in @gmx.co.uk or @btinternet.com and access it via Outlook).
Or are you using Microsoft's web-based email service, which you would most commonly access via your web browser and where your email address must end in @outlook.com (or the older @hotmail.com)?
(If it's the latter, which web browser are you using, please?)