Hi Madmaz140,
A good place to start in investigating an email concern is understanding how the Mail app functions. With the settings configured in your device, the Mail app will access your email account, check for messages, download content or send outgoing messages and then display the results for you to interact with.
Based on the error you are seeing, testing various network configurations would be advised. Try these actions on both cellular and Wi-Fi data, testing several Wi-Fi networks if possible. If you still see this across multiple connections, the problem may be in your device. If you have a VPN, ad-blocking, antivirus or other security-related softwares/apps/settings enabled, try turning those off or removing them and retest.
Regards.