If you can’t receive messages
If you’re certain you’re not receiving messages, even after clicking the Get Mail button 
in the Mail toolbar, try these suggestions.
Check whether Mail is retrieving a large message or a message with large attachments. For more information, see View Mail activity.
Check if you’re using filters in the message list that are preventing expected messages from being shown.
Try sorting messages in the message list by Date or another attribute, or searching for messages, to verify if you received them.
Check whether your account is offline or disabled (inactive).
Try quitting Mail, then opening it again.
Choose Window > Connection Doctor, then follow any instructions in the Details column. Check for problems logging in to email accounts or with the incoming mail server, or with firewall software blocking network traffic.
Check whether you’re affected by another firewall, such as software installed separately from macOS or built into an Internet sharing router, or administered by your email account provider. Contact the appropriate person for information.
If you’re logged in to a network using VPN, make sure that you entered your VPN password correctly.
Contact your email account provider to verify your user name, password, incoming mail server, and other required information; then verify the information is entered correctly in Mail account preferences.