Mountain Lion Update - Mail (SMTP) Offline Ever Since

Question

I have been struggling with an issues for about four days that seems to be a known problem.  Ever since I installed a Mountain Lion update the other day, my email servers have been offline (SMTP).  I have re-inputted all my settings, have checked them and double checked them.  I am completely lost.  I can see that other people have struggled with this issue and initially the advice was that perhaps it was a problem with the email supplier.  However, after doing my research I can see it seems to be a known problem with Mountain Lion, but I cannot find a fix anywhere.
I have an Apple Care call scheduled for tomorrow but given that I cannot find any definitive answers about this on these forums, I don't think Apple can supply me with one.
Has anyone had the same problem and successfully resolved?  I have tried changing the ports (all three numbers mentioned in other posts), I have re-inputted the details, I have tried the workarounds that have been suggested.  I'm not remotely techie but have tried everything I have read.  I cannot get either Tiscali or my work email servers back online.  I certainly don't want to change to Outlook or anything other than Mail.
Any ideas?  If Apple Care can help then I will of course post the outcome here
ps iPad and iPhone do not have the same problem - most bizarre, I can send emails from those devices, just not my Macbook with Mountain Lion!

Answer 1

Here's how to see if you're having the same issue I was having. (Note that, for me, the problem was not with my iCloud account; I don't know whether the steps below apply to fixing this problem for an iCloud account.)
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.pretendemailservice.com").
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.