That's either your local Mail.app configuration, your network, or something with whomever is providing your mail.
If you're using your network to post this, then that's working. Which then leaves either your local Mail.app settings, or something with whomever is providing your mail.
As a start toward troubleshooting this and if your mail provider offers it, see if your mail is available via a web interface. If your mail is available and working via the provider's web mail client (assuming they offer that client), then the next step is to verify your local Mail.app settings with whatever requirements your mail provider has established; those settings can and do vary by provider, and some providers have changed their own requirements.
If your provider offers support, call them. If not, you'll want to read the posted set-up details for your mail provider, and compare those with your Mail.app settings.
The Connection Doctor within Mail.app can sometimes help narrow down the problems. Mail.app menu Window > Connection Doctor, then open the drawer and re-run the tests. If you don't understand the output of the tests that gets displayed, then sanitize any personally identifying details, and post the output here.