This is something that seams to alway happen with Mac Mail no matter what version people have. This issue has been happening since 2006 and no other email program do this. I hope apple will fix this issue in up coming releases but I am not holding my breath as it been happening for 7 or more years.
Here are some steps that solve the issue for some of the people whom I had try it but there is no real fix other then deleting the account and resetting it back up. Please only do this with an IMAP account or you may lose your emails.
1. In Mail, go to the Mail menu and choose “Preferences…”.
2. At the top of the Preferences panel, choose “Accounts.”
3. In the sidebar, choose the account that you can’t send from.
4. At the top of that account’s panel, choose the “Account Information” tab.
5. Go down to the “Outgoing Mail Server (SMTP)” dropdown menu. Does the affected account’s smtp server have “(Offline)” next to its name in the dropdown menu? If yes, choose “Edit SMTP Server List…” from the bottom of the dropdown menu.
6. In the middle of the panel, choose the “Advanced” tab.
7. Can you see a Password field? If yes, is the Password field empty? If yes, then fill in the password and click “OK” at the bottom.
8. Now, as you did in Step 5, choose “Edit SMTP Server List…” from the “Outgoing Mail Server (SMTP)” dropdown and click on the “Advanced” tab. Is the password field that you just filled in empty again? If so, you’re having the same problem I was having.
Here’s how we fixed it on my computer:
9. Open your Applications folder. Find the Utilities folder and open it. Open the Keychain Access application.
10. In Keychain Access, in the panel on the left side of the window, click on “login.”
11. In the main part of the window, in the list of keychain items, locate the keychain item for the mail account that’s not working for you. Its name will likely start with “smtp” (as in, for example, “smtp.gmail.com”) or it’s possible that it could start with “mail” (for example, “mail.domainname”).
12. When you find the keychain item for the offline smtp server, click on it once to highlight it, then go to the Edit menu and choose “Delete.”
13. Now go back to Mail preferences and enter the password in the same place you tried to enter it in Step 7 above.
In my case, this fixed the problem. The password did not disappear after I clicked “OK,” and I was able to send email from the account again. I hope this works for you. I know how maddening this problem is.