You mean while using Thunderbird? This is an eM Client forum. But anyways:
- First check your credentials (username and password).
- Second, depending if you are using a POP or an IMAP protocol, check your server name, port number and security level.
Do some trials and errors. So, if you use port 587, set the security policy to Force usage of SSL/TLS
If your server uses port 25, try SSL/TLS if available
Or for port 465, Use SSL/TLS on special port (Legacy).