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 lightening 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 take an account online, if possible, or to see 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 see the account in Accounts preferences in Mail, double-click the text in the Details column.
To see log information while Mail tries to connect to mail servers, click Show Detail, then click Check Again.
When you’re done, click Check Again to see if your actions resolved the problem.
If the problem persists, contact your email account provider to verify your account details and setup.
To see 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.