Check that you specified when you enter the mail server username and password. For greater accuracy, write them in a text editor, check the data and copy them into the input window.Check the connection to the Internet. To do this, try to open any web page, which definitely should work. If no connection, then make sure that the provider paid, and check all settings. Sometimes mail does not work because restarting the servers or temporary technical problems. Wait a while and try again to connect to the resource.If you use Yandex, but the full version of mail does not work and downloading emails takes quite a long time, try a lightweight version of the website. You can try to fix the problem by refreshing your browser. Go to the developer of your browser by typing the program name in the search. Download and install the link "Upload" a new version of your program, and then try to log back on.When working with mail, try to use the HTTPS Protocol, which helps to fight corrupt data when transferring files from server to your computer. To go to work in safe mode, in the browser address bar to add "http:// the letter S to address started to look like "https://".If the receiving and sending of letters you use mail clients, check their settings. To do this, run your program, select the mailbox and click "Options", where you verify your information. If the mail log still does not work, then reinstall your app and upload the updated version from the developer's site. Otherwise try to use another alternative mail client or try to log into email account from a browser window.