Use Connection Doctor in Mail on Mac
Use the Mail Connection Doctor to check your internet connection or the status of your email accounts.
Tip: When an account or network is offline, a lightning bolt icon 
or a warning icon 
is shown at the end of the Favourites bar (just below the search field) or in the Mail sidebar. You can click the icon to try and take an account online or to view status information.
In the Mail app
on your Mac, choose Window > Connection Doctor.Review the status information.
If you’re connected to the internet, there may be a problem with the account’s mail servers or your account setup. Follow the instructions shown in the Details column. To quickly display the account in Accounts preferences in Mail, double-click the text in the Details column.
To show log information while Mail tries to connect to mail servers, click Show Detail, then click Check Again.
When you’ve finished, click Check Again to verify if your actions resolved the problem.
If the problem persists, contact your email account provider to verify your account details and setup.
If you contact Apple Support with Mail issues, you may be asked to select the Log Connection Activity tickbox in the window. When selected, diagnostic information is collected while you use Mail to help Apple Support understand and resolve Mail issues, investigate issues with your Mac, and help improve Apple products and services. The diagnostic information collected might contain personal information, including the content of your email messages. If sent to Apple, this information is used by Apple in accordance with its privacy policy and is not shared with any other company.
To display the folder of log files in the Finder, click Show Logs in the Connection Doctor window. After you provide log files to Apple Support, deselect the tickbox to turn off connection activity logging. You may want to delete the log files, as they can be large.
Learn more about the Apple Privacy Policy at apple.com/uk/privacy.