Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Configuring your Mac Mail Client
To configure your Mac Mail client you’ll need to follow a sequence of easy steps and specify the following parameters:
• Full Name. Your identifier seen by the recipient in the From field of your emails.
• Email Address. You email address that includes both the username and domain.
• Password. A unique password that will protect your emails.
• Account Type. Mac Mail offers three mail protocol types – IMAP, POP, and Exchange; you should choose the preferred one.
• Description. Unique description of your email account.
• Incoming Mail Server. Address of the server that will receive your mail, set in the form of mail.domainname.com or just domainname.com.
• User Name. Same as your email address.
• Outgoing (SMTP) Mail Server – Address of the server that will send your mail, set in the form of mail.domainname.com or just domainname.com. If your outgoing server requires authentication, you should select the corresponding option.
Once the parameters are set, Mac Mail will check connection to the specified mail servers and show you the summary info for your account. Check whether the info is correct, and click Create. Finally, send a test mail to your own mailbox to make sure you’ve made everything correctly.