It just happened recently. I used to be able to receive messages, and then i started changing things to be able to send messages but no i can't send or receive anything.
Assuming you're using the standard Mail client, I would recommend that you first review your settings and ensure that they match what we've got here: How to set up email in Apple macOS Mail
Also, you'll want to ensure that your account is online as well by clicking the Mailbox menu item at the top and clicking Take All Accounts Online.
Hope this resolves the problem for you, but let us know if you're still having any issues sending or receiving. Any error messages you receive will be helpful.
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